I started typing below but wonder if there are any objections replying in English on this German sub forum ?
Make sure all the DNS services that you depend upon are chained so the DNS path for resolving hostnames to IP addresses would look like so:
[Clients] --> [Pi-Hole] --> [Fritzbox] --> [ISP DNS servers] --> [Root DNS servers]
The result is that if Pi-Hole cant resolve a DNS query from one of the clients,
it will ask its upstream configured DNS server which is Fritzbox.
And if Fritzbox cant answer the client query, Fritzbox will ask its upstream DNS server(s) from your ISP etc etc.
EDIT: But be careful setting a secondary DNS server setting alongside a primary one!
If you setup one of your DNS services to use a secondary DNS server upstream, make sure both primary and secondary will resolve to the same DNS records.
If they dont, you can expect it sometimes working OK ... and sometimes not.