Private Dns is working fine on WiFi, as the title says. However, "some sites and apps" just don't work on cellular network. For example, nextdns.io works, but my.nextdns.io and the help page both give ERR_ADDRESS_UNREACHABLE.

Even the Uber app doesn't work on cellular network. You will be stranded on the road, if you didn't figure that it's the private Dns at fault. 

The nextdns logs also have no entry of me attempting to reach these domains or apps. 

If I switch off private Dns, all of this starts working. It's not like my cell network has issues with nextdns, else my.nextdns.io should not resolve when private Dns is off.

Even the nextdns app, when enabled, exhibits the same behaviour.

Additionally, it's not anything to do with my block list. As everything works smoothly when connected to WiFi with private Dns 'on'.

I have witnessed this problem on all new and old android phones of family members at home.