Update: I got it working, kinda.
Bought a Fritzbox 7590PN, it has presets for all the weird things that providers do. The “modem installationscode” did not work, but username + password worked.
username: vodafone-vdsl.komplett/vbXXXXX
password: the password
I had changed the password (absolutely no reason to do that, they store it in plain text) so I needed to use that.
Now I have the issue that the FB only has 1Gbps LAN ports, i.e. 125 MB/s which is half of what I have.
But if I dont want to use the FB itself…
And disabling wifi on the FB works easily, prioritizing the MOX in LAN works, and the MOX works too.