As I didnt find any appropriate thread to continue ...
I got a quite antique T-Mobile Speedstick LTE III (Huawei 12d1:1506)
and after a while of muddling around with modeswitch and smstools, I was able to send an sms
But then I got IO ERRORs on the machine, IP stack was still there, but all services, and GUI, fell down (incl sshd) and I had to reboot the box the hard way.
It didnt come up again, until I removed the Speedstick.
So this thing is the definitive cause Ubuntu 18.04 (OK, it's a Mint Tricia :-p ) will hang after a while, and at startup.
Any idea if this is fixable?
( I mean, buying another LTE modem, yes ... but besides that? )