@lduboeuf My mobile data are working since yesterday!
It seems that the problem was an ESIM from the previous owner which I found when I reinstalled Android. There it was activated, so I deleted it and then installed UT. And now the mobile data are working.
Thank you all!
Interestingly, over the last 2 weeks I got most of the days mobile data, but after turning on flight mode or a power cycle the mobile data were gone for the day.