I have installed Pi-Hole on a Raspbian GNU/Linux 10 (buster) system, running on a Raspberry Pi 4. I installed everything carefully following online instructions, without really knowing what I'm doing... but I've managed to get everything working, and i even managed to get Kodi working on the same Raspberry pi machine. My router is D-Link DIR-825ACG1.
Last week something got messed up, don't know why and how, but this is what happened. I was away from my house for a long weekend, so I disconnected all the electrical appliances, including the Pi and the router. When I re-connected everything, I couldn't get the internet working on my wi-fi network.
When I re-enabled the DHCP server on my router and disconnected the Pi from the network, the internet connection started working as expected again, obviously without the Pi-hole blocking ads etc.
Another thing that happened, is that the Raspberry Pi can't connect to the internet. Obviously I try that when the Pi is connected directly to the router with a cable. I can access the router's admin panel or the pi-hole admin interface through the Pi's browser, but can't access any other pages. Kodi is also not connecting to the internet, and another indication is that the Pi's system clock can't sync with an online server as before.
Currently I disabled the Pi-hole's DHCP server through the web interface, and re-enabled the DHCP service on the router.
I've put the text here -
Mind you - the time stamp is wrong, because, as I mentioned, the Pi can't sync to a time server and shows the wrong time and date