NetworkManager doesn’t support DoH, DoT or other recent protocols like DoQ and DoH3. You’ll need to set up a local DNS resolver / proxy which can handle those protocols. You could use dnsproxy for this. Once you set it up, you can just use “127.0.0.1” as your DNS server in NetworkManager.
Btw, if possible I’d recommend sticking to DoH3 (DNS-over-HTTP/3) or DoQ (DNS-over-QUIC) - they perform better than DoT and vanilla DoH, and are more reliable as well.