FluidNC supports both TCP/Telnet connections via port 23 and WebSocket connections via port 80 (>= v4.0) and 81/82 (< v4.0). UGS supports connections via either TCP or WebSocket but I believe MillMage only supports the TCP connection. So, you would need to use port 23.
I was able to successfully connect MillMage to my FluidNC machine using port 23 but not 80/81/82.
These are the default ports anyway. They are configurable to use different port numbers.