I also blocked port 53 in my firewall (except for the Pihole; no DoH there). After that, I noticed that some applications have some DNS servers hard-coded. 8.8.

I also blocked port 53 in my firewall (except for the Pihole; no DoH there). After that, I noticed that some applications have some DNS servers hard-coded. 8.8.8.8 being pretty prominent. My solution was to assign the Pihole the IP address 8.8.8.8 as well. Then I added a static route in at the router to route 8.8.8.8 to the Pihole. Now every request to dns.google will also be handled by pihole instead of getting timeouts.