[Bug]: T56 stays on 3G with weak signal; provision roaming and validate LTE readiness #11

Component

Android radio application / device provisioning / cellular modem integration

Version or commit

Investigation baseline: e3657bf780fd.

Record the exact installed APK version, build, and baseband when the T56 is connected over USB.

Affected hardware

Primary report:

• T56 — UNIPRO / ZX, Android 5.1.1 / API 22

Compatibility and regression scope:

• T99
• RYKS
• Generic Android

Summary

After inserting an active SIM into the T56, the device remains on 3G and reports persistently weak signal strength even though the test area has confirmed 4G/LTE coverage.

Provisioning currently does not explicitly establish and verify a cellular-readiness policy. For managed radio deployments, Data Roaming should be enabled from the beginning of provisioning and verified again after reboot. This is required because some managed, MVNO, multi-network, or cross-network SIMs need roaming permission even when used within the intended service area.

Enabling Data Roaming is one required preventive measure, not a complete diagnosis and not a guarantee of LTE. The investigation must also cover preferred-network mode, APN/carrier configuration, SIM entitlement, modem firmware, LTE-band compatibility, actual serving-cell availability, and Android/Minimum telephony reporting.

Current behavior

• The SIM is detected and mobile service is available.
• The network remains reported as 3G.
• Reported signal strength remains low.
• The behavior persists in an area where 4G/LTE is otherwise available.
• Provisioning does not currently report a verified cellular-ready state.

Expected behavior

• Provisioning enables Data Roaming by default for supported managed-radio profiles before the final connectivity checks.
• Provisioning sets or verifies a safe automatic preferred-network mode that includes LTE and legacy fallback; it must not force LTE-only operation.
• When the SIM, operator, LTE bands, and coverage are compatible, T56 registers on LTE.
• Android system state and Minimum telemetry report the actual active data network and a valid signal value.
• The configured cellular state survives reboot.
• Model-specific workarounds are gated to verified hardware and do not regress T99, RYKS, or Generic Android.

Provisioning changes

1. Enable and verify Data Roaming

• Make Data Roaming enabled by default in the supported-device provisioning workflow.
• Apply the setting early enough that cellular registration can stabilize before readiness checks.
• Make the operation idempotent.
• Read the value back after writing it; do not report success from command exit status alone.
• Verify the value again after the provisioning reboot.
• Print a clear operator notice that roaming can incur carrier charges.
• Provide an explicit opt-out for deployments whose SIM policy prohibits roaming, while retaining default-on behavior for the managed workflow.
• Handle Android-version, OEM, and subscription differences explicitly; fail or warn when the setting cannot be verified.
• Avoid exposing subscriber identifiers in console output or logs.

2. Establish a safe preferred-network policy

• Capture the original preferred-network mode before mutation.
• Detect whether the current mode allows LTE.
• Select an automatic LTE-capable mode appropriate to the verified modem/firmware and subscription, with 3G/2G fallback.
• Do not use LTE-only mode because voice, emergency, or coverage fallback may depend on legacy radio access.
• Read back and report the effective mode before and after reboot.
• Do not apply numeric preferred-network constants blindly across Android/OEM variants.

3. Validate APN and carrier readiness

• Confirm that a usable APN is selected for the active SIM/operator.
• Validate APN type, protocol, roaming protocol, authentication requirements, and carrier enablement using sanitized output.
• Do not overwrite carrier-provided APNs indiscriminately.
• If a managed APN is required, make its configuration explicit, model/carrier-scoped, idempotent, and testable.
• Detect SIM-not-ready, data-disabled, carrier-restricted, roaming-denied, and LTE-not-provisioned states separately.
• Record whether the SIM is an MVNO or uses a partner network without publishing IMSI, ICCID, phone number, or credentials.

4. Add a cellular readiness gate

Provisioning should not declare cellular readiness only because a SIM exists.

• Wait for SIM and service state to stabilize within a bounded timeout.
• Report voice network type, data network type, roaming state, operator name, data connection state, and signal unit/source using sanitized values.
• Confirm that mobile data is enabled and a usable cellular route exists.
• Prefer LTE when compatible and available, but accept documented 3G/2G fallback rather than looping indefinitely.
• Distinguish PASS, WARN, and FAIL outcomes with a useful reason.
• Re-check after reboot.
• Preserve the device's existing app data and Device ID.

USB/ADB investigation plan

When T56 is connected:

• Capture sanitized manufacturer/model, Android build, baseband, modem properties, and installed Minimum version.
• Capture SIM state, service state, data/voice network types, roaming state, data state, preferred-network mode, and selected APN.
• Compare the system status UI, TelephonyManager, signal callbacks, and sanitized dumpsys telephony.registry evidence.
• Record evidence before changing roaming or network mode, then compare after each isolated change.
• Toggle airplane mode once or perform another controlled radio re-registration, then wait for stabilization.
• Reboot and confirm persistence.
• Test with a known-good LTE SIM in the T56.
• Where practical, test the affected SIM in a known-good LTE device at the same coarse location.
• Verify the T56 modem's supported LTE bands against the operator's serving bands.
• Determine whether weak signal is real or caused by stale/invalid values or incorrect ASU-to-dBm conversion on Android 5.1/Qualcomm.
• Record whether the observed 3G label comes from the modem, Android framework, system UI, or Minimum/APRS telemetry.

Do not upload a full bugreport or raw logs containing IMEI, IMSI, ICCID, phone number, device serial, APN credentials, access tokens, certificate fingerprints, exact private coordinates, or other unique/private data.

Additional preventive fixes

• Add a reusable sanitized cellular diagnostic command/report to the provisioning bundle.
• Capture before/after provisioning state so unintended modem-setting changes are visible.
• Detect unsupported LTE bands or firmware limitations and report them as a hardware/carrier limitation instead of repeatedly rewriting settings.
• Use bounded recovery: one controlled re-registration/reboot, then stop with evidence rather than retrying forever.
• Validate state after SIM replacement because carrier configuration and roaming behavior may change.
• Ensure provisioning does not silently disable roaming or LTE during later upgrades.
• Document the managed cellular policy, operator cost warning, opt-out, and rollback steps.
• Add automated parsing/policy tests without relying on private device identifiers.
• Keep signal/network telemetry unavailable when Android returns unknown or invalid values instead of presenting misleading weak-signal data.
• If runtime telemetry is shared, prefer API/capability-based behavior; gate Qualcomm/T56-specific handling by the verified device profile.

Cross-model validation

• Identify which roaming, APN, mobile-data, network-mode, and signal-reporting code paths are shared.
• T56: real-device cellular acceptance required.
• T99: verify provisioning and reboot persistence; record cellular capability and test evidence.
• RYKS: verify provisioning and reboot persistence; record cellular capability and test evidence.
• Generic Android: confirm no unsafe OEM-specific setting is applied and telemetry degrades safely.
• Do not claim LTE-band or real-signal acceptance for any model without physical evidence.
• Unknown or ambiguous hardware must be rejected before model-specific mutations.

Acceptance criteria

• Root cause is documented with sanitized USB/ADB evidence.
• Supported-device provisioning enables and verifies Data Roaming by default, including after reboot.
• The provisioning output includes a roaming-cost warning and an explicit opt-out.
• Preferred-network mode safely includes LTE and legacy fallback where supported.
• APN and SIM/carrier readiness failures are diagnosed distinctly.
• T56 registers on LTE under confirmed compatible SIM, band, entitlement, and coverage conditions, or a verified hardware/carrier limitation is documented.
• Minimum reports the same active network generation as Android/modem registration state.
• Signal telemetry does not remain falsely low because of stale, invalid, or incorrectly converted values.
• Genuine 3G/2G fallback continues to work.
• Cellular settings persist after reboot and remain idempotent on repeated provisioning.
• Shared behavior is covered by automated tests where feasible.
• T99, RYKS, and Generic Android regression impact is assessed and recorded.
• Real-device results clearly distinguish tested facts from assumptions.

Test record to complete

• SIM/operator: [sanitized operator name only]
• SIM type: [direct operator / MVNO / managed multi-network]
• Coarse coverage comparison: [no exact private coordinates]
• Installed Minimum version: [enter]
• Android build/baseband: [sanitized]
• Roaming before provisioning: [on / off / unknown]
• Roaming after provisioning and reboot: [on / off / unverifiable]
• Preferred-network mode before/after: [sanitized symbolic values]
• Selected APN status: [sanitized]
• System/modem data network type: [enter]
• Minimum/APRS network type: [enter]
• Reported signal: [value, unit, and source]
• Known-good SIM/device comparison: [enter]
• T56 result: [not tested / pass / warn / fail]
• T99 regression: [not tested / pass / warn / fail]
• RYKS regression: [not tested / pass / warn / fail]
• Generic Android regression: [not tested / pass / warn / fail]

Safety

• Enabling roaming may incur charges depending on the SIM and carrier agreement; provisioning must show this warning.
• Do not transmit over PTT while gathering cellular diagnostics.
• Do not clear Minimum app data.
• Do not alter device or subscriber identifiers.
• Do not publish secrets, subscriber identifiers, exact private coordinates, or unredacted configuration.