Mobile internet via cellular network mostly not working

[grump_fiddle_reinstall]

For some time now (since Bookworm? Or only a few weeks? I actually don't know) I usually don't have internet access via the mobile network.

Sometimes it works, mostly it does not. A restart usually helps, but of course that's not what you want to do when you need to quickly look something up or when you or wait for a Signal/Telegram message while on the move. The problem occurs with two different providers.

Is there anything I can do?

I noticed tonight that my 4G data is no longer working. The 4G connection seems to be up and there is plenty of data on my plan. Rebooting and updating have not helped.

Prior to this 4G data has been pretty solid with only the occasional reboot to get it up and running again.

Edit ; I will try the resolvconf fix above as the modem is reported to be working but nothing resolves.