Good evening!
A solid week of running has proved that the 100MB limit was the issue. The new SIM that isn’t limited hasn’t killed a session once. It has remained online and happily throwing our data around without a problem. Several of our containers have shown issues, but that’s a different story and nothing to do with the connectivity.
So, we’re not too sure of the exact procedure needed to manually start a new session (though they said that the SIM would automatically start up a new session for us) - but a very brief attempt was made at restarting the modem if it failed to connect for any reason.
Using mmcli, restarting the modem is as easy as an mmcli -m 0 -r or something rather similar, and worked well. Once the modem came back up and reconnected, it did so with a new session (presumably, as it would start working again).
Using DBus in our networking container, hitting the modem interface with a reset just seemed to kill off the device as a whole and a reboot was needed to have it come back up. So that was pretty strange.
However, now I’m tangenting into something else, slightly.
So, sadly no immediate nice fix for anyone to copy (we sledge-hammered a device reboot in there if it’s been offline for over an hour, though, so there’s that), but can confirm the particular cause of our version of this error and a pretty good fix of simply moving to a different SIM provider.