I've had this problem since PiHole 4 and I was wondering if there was a solution.
I have a Velop router (consumer grade) which my RPi4 connects to via ethernet. I run unbound on the Pi as well as PiHole (also WireGuard but I don't think that has anything to do with this). PiHole is also my DHCP server. If I ever reboot the RPi4, it disappears from my network. As in no SSH, no PiHole web access, no pings, nada. As if it's physically unplugged. Obviously, the whole house then loses internet connectivity. The only way to fix this is to turn on the router's DHCP server, wait a while and then my RPi4 re-appears. Then I have to turn the router's DHCP server off again. Everything works fine until I reboot again (which I try to avoid).
What could be causing this?