Weird that you have to do an extra step for IPv6. Other ISPs in Germany have enabled it for every customer at some point. Unless your router asks for IPv6 addresses, nothing really changes anyway. So maybe just enable IPv6 on your router and see what happens?

On a side note, there seem to be ways to get out of CGNAT when you got condemned to use it: It is sometimes an annoying source for client VPN instabilities and from what I heard, users can just ask to be switched over from DS-Lite to classic dual stack to improve application compatibility.

Must be an old contract, all new contracts appear to be CGNAT/native IPv6 across ISPs