Disconnections with 2G on Meteo-40 data logger

Hello,

When I connect a Cinterion PLS8-E to an Ammonit Meteo-40 data logger here in Berlin, I get an LTE connection. If I run mmcli --modem=0 --set-allowed-modes=2G, the modem reconnects to the network with EDGE. Some activities work on the network, like opening an SSH connection or pinging. However, if I attempt something that requires more bandwidth (e.g. HTTP), nothing gets transferred and the modem eventually vanishes from ModemManager until I power cycle it.

We've had some intermittent problems in the field (e.g. fine for one week, premature disconnections another week, fine again, ...) and we suspect the root cause might be that 4G is not available (or isn't being selected for whatever reason) in those periods, which is why I'm trying to test with 2G.

We're currently using 1.14.12, but I compiled 1.18.12 and reproduced the issue. Here are the debug logs from NetworkManager and ModemManager 1.18.12. I connect the modem, set it to 2G, then attempt HTTP until it no longer shows up in mmcli -L.

If I do the same on my debian bullseye PC, the modem works well (but slowly, of course). It never disconnects. I have no idea what the data logger might be doing differently. Do you?

At the moment, we don't call SetCurrentModes anywhere. Our short term plan is to call that to prefer/ require 4G (if the user configures it). Is that helpful? i.e. are there circumstances where 4G is available but would not be selected by default?

By the way, the disconnections in #564 (closed) behaved in a similar way. One week I could reproduce it very reliably, but another week I couldn't reproduce it at all. It might be related.