You should post your question on the German section of this forum.
Edit: Seems a moderator moved it. Thanks!
Are you trying to connect to 5G or 2.4G? If 5G then change the channel to e.g. 40.
For some reason, these connect boxes don’t seem to transmit a country code so Macbooks often find one from a wireless point nearby, which is set badly, and then limits the channels it attemts to connect to.