Is that even legal? I always thought a land-line connection is mandatory.

I recommend to check the law here :)

I don’t think there is any such law, not that I could find. Telekom refuse to make the connection. They say that if it isn’t there when the apartment is built a new one can’t be added. I live in an attic conversion

I just checked. There is no right to a land-line but a general right to being connected. So somebody has to provide you a connection. If nobody does you can message the "Bundesnetzagentur" and they will oblige a provider.

https://www.bundesnetzagentur.de/DE/Vportal/TK/InternetTelef...