Connect Cinterion modem with SWWAN fails with unknown error

[Sven Schwermer]

Hi again,

Please excuse the continuation of the monologue, but I believe I have found a good hint if not the reason for the issue at hand. I noticed two things:

1. When NetworkManager wasn’t running on my device, I couldn’t establish the connection manually.
2. When I talked to the native UART without the USB port connected, I couldn’t establish a connection manually either.

This makes me think that the modem expects some sort of interaction with the network interfaces while connecting with AT^SWWAN. I think this may be the DHCP configuration. Coincidentally, the SARA-U2xx and LARA-U20x required the same thing: https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/50 <https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/50>

I have yet to find this behaviour described in the documentation.

Sven
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