FritzBox Configuration: Turn on IPv6
To make the Fritzbox IPv6 capable (on an appropriate connection of course) just go to
network -> network settings -> further settings -> IPv6 configuration
and enable the "DNSv6 Server" and the option "Assign IPv6 Server and IPv6 Prefix (IA_PD)".
This way the FritzBox updates the address prefix of connected devices when it is renewed by the provider.
If you want to access the FritzBox from outside, for example via VPN, you have to activate "PING6" and "Open firewall for delegated IPv6 prefixes of this device" under Internet -> Shares.
Experience has shown that a restart is always advisable when making these changes, otherwise the settings will not be applied!