PDP settings for LTE only network

[Brendan Simon]

Hello,

I have a number of embedded linux boxes with Quectel EC21 4G modems,
connected to various cellular networks (AT&T, Verizon, ...).

AT&T are removing 3G from some areas and the modems are not reconnecting.
My devices are vanishing off the face of the Internet.

Quectel have provided some assistance and guidance and advised to configure
some settings for AT&T Sunsetting.  The UE Usage Setting was changed to
configure the device as Data Centric (i.e. no voice), however it did not
resolve the issue.

AT&T provided some assistance - apparently the APN was wrong.  Seems like
some generic APNs are being used and not the APN specified in the
NetworkManager configuration.  This all worked ok when 3G was available.

AT&G also suggested the PDP settings of the modem are the issue.
Changing the default PDP context (#!) from an APN of "" to the provided APN
seems to have resolved the connection issue.

I have used `mmcli -m N --command="..." to modify the PDP context for
testing.

I feel this is hacky and shouldn't be required.

*Is there a better (more elegant and generic method) to set the PDP
contexts so modems (preferably any brand) will work on 4G only networks?*

*NOTE: we are using Debian Buster with ModemManager 1.10.0.*


*Would upgrading MM help?  Buster-Backports has 1.14.0 available, which
would be easy to build/install on the existing OS.*

Thanks, Brendan.
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