I had one of these boxes and found it to be beyond infuriating

I would set up something simple like port-forwarding to a static IP and test that it worked

then I'd come back a few days later to use it and found the router had helpfully changed the IP to another one

and this happened with several different features (IPv6, DHCP, etc)

I replaced it with a much cheaper Mikrotik box and that's worked flawlessly ever since

I would not recommend the Fritzbox to my worst enemy