@Fresh
Thank you for the explanation, csRegstatus can then also be used for Nb-IoT, next to Cat-M1 right?
I have implemented the changes, setting MNO to 19 as well as setting the priority to CELLULAR_RAT_M1.
Now for the first time, I was able to run for 24 hours without running into problems. I will keep running for about a week to make sure there are no other possible bugs. I will report back here later.