@kugiigi I'm sorry for providing the wrong answer yesterday. It's the same like in your picture: Activating cellular data brings the dots in the Wifi icon.
@lduboeuf Never used that too and output of

systemctl --user status batterysaver.service

is

Unit batterysaver.service could not be found.

@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.