I would recommend setting the PiHole as your DHCP server, my router wouldn't even use the DNS I provided so that's what I ended up having to do. ( I have SagecomF@ST 5260 ) I'd recommend this over diagnosing weird router behavior but it's up to you
To do that you have to:
- Make sure your PiHole has a static IP configured in
/etc/dhcpcd.conf
- Enable DHCP in the Pihole
settings
->DHCP
tab - Disable your routers DHCP server ( likely under the DHCP tab there )
- ( if you turned your Raspberry PIs firewall on you might have to open some ports )
You can find more info here