Hey all together! First of all thanks to the community for creating, maintaining and developing this cool service 
I like pi-hole so far, but there is something odd in my configuration.
The issue I am facing:
Clients work well for a while, but then loose connection/are not routed via pi-hole anymore. I couldn't see a pattern why/when this happens and which devices preferably
My gut feeling says there is some weird interplay of my fritzbox 9561 cable configuration and pi-hole.
Details about my system:
Pihole as LXC on proxmox. Fritzbox 9561 cable as router and modem. I should have a static IPv4 and IPv6 by my provider (Telekom, but via cable...). However I'm going to backcheck the current status of that. In the following I'm going to provide screenshots of my fritzbox settings and some thoughts about it. To set up my fritz.box, I used this guide:
I had the cloudflared https stuff installed, however I've disabled since I enabled unbound. Currently neither is running to make debugging easier.
1. Local Network setting
1.1 IPv4 Settings
192.168.178.130 is my pihole. First issue I'm facing: Setting my local DNS server to my gateway, as in screenshot, will cause that I can't access local devices via my local DNS, for example klinke.eisbein for the pi-hole container. This happens mainly on my mobile android device. Local DNS works currently for my pc (192.168.178.102/103) even though it's not connected. If I set 192.168.178.130, so my pi-hole, the fallback to public DNS servers does not work, which would be crucial feature of my home network.
1.2. IPv6 Setting:
Here i just followed the guide. I have the feeling, this could be some root error - since my internet provider offers me IPv6. (I'm not too much into network, hope you get what I mean)
2. "Internet" Settings
2.1 IPv6 support
I wanted to disable IPv6 to make my life easier. However, this caused problems. Either no internet connection or no client connection with pi-hole.
2.2 DNS settings
I guess that should be clear...
2.3 Parental control/filter to force using pihole
(images see reply)
I followed the guide to setup a filter, that forces clients to use pi-hole (?). Pi-holes access profile is unrestricted, all others use "standard" which has "blocked" udp and tcp.
What I have changed since installing Pi-hole:
I focused the same issue from beginning. With the current settings, it works better, so for a longer timerange, however not persistent.
The debug log token is Ifj0kUwk. I hope it's fine to have focused my post content on my fritzbox. Please tell me if any other information is needed and can't be gained from the debug log.
