Pi-hole is intended to run as a filtering DNS resolver within the bounds of a private network. As such, it listens on the standard DNS port 53.
Android's Private DNS employs DNS over TLS to talk to a public resolver over an encrypted connection via port 853.
If you want to use Android's Private DNS feature with Pi-hole, you'd need a DoT proxy that would accept and decrypt requests on port 853 and then forward them to Pi-hole on port 53.