Most likely, you have setup your Pi-hole as your Fritzbox's upstream router (under Internet|Zugangsdaten|DNS-Server ).
While this filters all DNS traffc in your local network, inlcuding Fritzbox guest WLAN, Pi-hole will see all DNS requests as originating from a single machine: your Fritzbox.
To change this, either
- disable upstream DNS resolution and switch to local DNS-resolution
or - just add local DNS-resultion
By handing out Pi-hole as local DNS server to your clients, clients will ask
your Pi-hole directy, allowing Pi-hole to see the correct origin of DNS queries.
For a short guide how-to do this, search this forum.
If you can read German, a link to one of my previous posts might suit you as well: Verzweiflung pur - Lange Verzögerungen & Ladezeiten oder gar keine Funktionen - #13 by Bucking_Horn
It's focused on just local DNS resolution, though.
When reading, Try clicking the black arrows before some of the post's head lines
to expand detailed instructons, just like here 