TLDR; Try to just block the band the Mudi is connected to, when the issue occurs and leave all other bands unblocked.
There is a known issue with registering the sim with the tower with certain bands. I’m having the same issue with a Deutsche Telekom Sim. For me, the connection worked only occasionally, because my Mudi changed bands. I then blocked band 1 which wasn’t properly connecting, with the masking feature, but left all other bands active and things have been working fine for me since.
I am in contact with the support over the issue and they are actively working on resolving the issue with a future update. There is an special thread just for this issue as well: SIM Registration Suggestions of Mudi7/GL-E5800 for Parts of Europe