Pi-hole X2

My broadband router has entries for two DNS servers, primary and secondary. I entered the IP address of my raspberry pi running pi-hole as the primary and entered Google's DNS server's address as the secondary. Ads would be blocked for a while but then, for no apparent reason that I can fathom, the router would start using the secondary DNS server and ads would start to leak through. Since I had a second raspberry pi not being used, I installed pi-hole on the second pi and began using that at the secondary DNS server. No more ads!

Many devices don't distinguish between primary and secondary DNS servers. They will use whichever one they want.

I lied to my router by giving a bogus IP for the second DNS slot without any ill effects. The fake IP is pretending to be on an unprogrammed, thus invalid, subnet. The router dutifully tries them both and since one doesn't work, it stops trying pretty much immediately. Eventually I set up a second pihole and used that IP instead. Even still, the second pihole doesn't see much use.