Fix for Dragonfly nano and registration for LTE-M plans

[pferland-multitech]

Description

Summary of change

The cellular_properties object for the SARA4_PPP instance did not allow CEREG to be used for registration checks. LTE-M plans require that CEREG is used to check for registration as its separate from 3G or 2G service (CREG/CGREG respectively). CGREG is left in because the radio has 2G fallback.

In current LTE-M dataplans in the US (on Verizon and AT&T at least) CEREG will report a successful registration while CGREG and CREG will not. This means that the cellular connection will not be established as mbed does not think the modem is registered.

Documentation

None.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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Release Notes

Summary of changes

Changed cellular_properties object for Dragonfly Nano.

Impact of changes

Registration with LTE-M only data plans should now work.

Migration actions required

None

[ciarmcom]

@pferland-multitech, thank you for your changes.
@ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[AriParkkila]

If AT+CREG is supported it should be enabled as well.

[0xc0170]

If AT+CREG is supported it should be enabled as well.

@pferland-multitech Please review

[0xc0170]

@pferland-multitech Any update?

I'll start CI meanwhile as it was approved

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[pferland-multitech]

AT+CREG will return a response, but as the modem is LTE-M or 2G only AT+CREG should never respond with a registration on this device.