Hi fellas,
I've read a lot of topics here and on reddit about this common problem, unfortunately nothing solved my issue.
I go direct to the point: since I setupped my raspberry pi with Pi-Hole and leave my Fritz as a DHCP server my wireless clients (sometimes, maybe after 2/3 hours of works) stop working! The wired connection does not suffer of this issue, just wireless. The clients are connected but without internet connection.
I tried a lot of solutions: moving the DHCP from Fritz to Pi-Hole, disabling IPv6, using Google DNS, changing router settings, re-installing Pi-Hole, re-installing PiVPN (maybe it was related!), setting iptables... same issue 
I dont think i'm getting anywhere.
The guide I followed (step by step) to setup Pi-Hole is -> Fritz!Box (EN) - Pi-hole documentation
I dont know where the problem is, considering the wired connection is still working great!
It's weird because if I reboot my router, the connection back to work and all my wirless clients have connection... but after 2/3 hours.. it stops again!
Below my token
https://tricorder.pi-hole.net/hpX6Xpn0/
If you have an idea or suggetsion for my issue, please gimme a shout 
It's much appreciated!