My Pi-hole setup is probably a little unusual so I thought I'd share it.
I wanted to be sure that even if the Pi fell over, I'd still be able to access the internet and be clean of adverts. If I told my router just to use one Pi-hole for DNS then if it stopped working for any reason then the internet would fail. If I told my router to use a public DNS server as a secondary then if the Pi stopped working then everyone would get adverts.
I built a second Pi-hole. The nice thing about Pi-hole is that it runs perfectly well on a Raspberry Pi Zero so I used one of those.
My main internet router has four ethernet ports.
Two of them go to switches, one goes to my network monitor and there's nothing in the final port.
My two Ubiquiti access points are connected into different switches and I have one Pi-hole per switch as well.
If either Pi fails, all DNS requests will go via the other. If either switch fails, all the wireless devices will connect to the othr access point and use the other Pi-hole. My network monitor will email me to tell me what device(s) have failed.
Obviously if the main router fails then eveything stops working, there's nothing I can do about that but I do have a third Pi monitoring my overall internet connection so if it can't access the internet, it force reboots the router.
It's not a bad setup