I have tried multiple sims with the GL-E750 and none work. I get no internet connection. Does anyone know if there is a network compatibility issue with this device or a settings issue? Also it only seems to work with T-Mobile on the US. Verizon doesn’t work and I haven’t tried AT&T. For a Global device cellular compatible seems to be lacking.
Quick update: The error I get is -251 when using the connection wizard with a Smart Telecom sim.
GSM Error Code 251 isn’t standardized 
Do the SIM work when you use them in a phone?
Yes. They work with no issues in 3 generations of iPhones: 10, 12 and 13.
Since I don’t own a Mudi I am just trying to bring light into the dark - so please don’t take me as an expert here:
Would it be possible to set up cellular manually without the connection wizard?
You should see all necessary details on your iPhone: View and edit your Access Point Name (APN) on your iPhone and iPad - Apple Support