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Expected Behaviour:
Pihole is running on a Pi Zero 2 W using Raspberry Pi OS Lite 64-bit (Debian Bookworm) connected to my AT&T router/modem via WiFi. A static IP is assigned via the router's DHCP pool for when it's on, and also the Pi itself has a static IP configured via nmtui for when I have DHCP on via the Pihole. Nothing else is running.
I have the Pi-hole configured as the DHCP server for my network with the router's DHCP off, static IP. I'd expect that devices on that network would resolve DNS successfully via the Pi hole.
Actual Behaviour:
Initially, that's what happened, but then at some point it began refusing all requests from what I can see in the query log, both on multiple other devices and even for the Pi hole device itself. I tried disabling the rate limit, and when that failed doing a wipe and fresh-install, but the issue persists.
Quick note on this: I'm not sure why, and I haven't tried long enough to conclude if it's coincidence or not, but disabling OpenDNS and only using one upstream (Cloudflare for me) seems to have resolved the issue. It comes back immediately if I enable OpenDNS upstreams and restart the system. I guess I have my workaround.
I've fiddled more with other settings, and I'm now unreasonably kind of sure that the issue is with my AT&T fiber router configuration or something with how AT&T locks down the router, and not with something about OpenDNS.
I can have DHCP off on the AT&T router (it splits IPv4 and IPv6 into two toggles, with IPv4 being the one generically labeled 'DHCP') and on for the Pi Hole and have everything work fine, but if I turn on IPv6 support, even if I disable the Router's DHCPv6 and IPv6 capability (I'd read elsewhere that leaving IPv6 on still leaves the router to advertise itself as an IPv6 DNS server), it causes everything to get refused again, both IPv4 and IPv6.
That said, even if I disable IPv6 support on the Pi Hole and re-enable IPv6 support on the router, enabling or disabling OpenDNS does still break/resolve the issue as described initially, so I'm at a loss for what that means. Maybe ghosts, probably I'm misunderstanding some step or how long it takes for something to 'update', or a sign I just need to go the passthrough router route.