I have an Asus RT-AC5300 router. I have a Windows 10 PC, a Windows 2016 server machine, and an Android phone.
Using a remote desktop client on the phone, I can connect to both computers while on the same TCP subnet (i.e. via Wi-Fi). I can connect to the Windows 2016 Server machine from a remote location, by connecting to (e.g.) a.b.net:8389 because I have mapped virtual server TCP/UDP port 8389 in the router to port 3389 on the Windows 2016 machine's IP address.
However, I have also mapped the router's virtual server TCP/UDP port 8489 to port 3389 on the Windows 10 Pro machine's IP address - the same address I can use to connect while on Wi-Fi - and in the Advanced Firewall settings, I have enabled edge traversal for all RDP-related rules on all network types, but when I attempt to connect to a.b.net:8489, I get the same error as if I tried to connect to an invalid port on the router.
As far as I can tell, Port 8489 on the router is not used for any other service/mapping.
How can I connect my phone directly to the Windows 10 Pro machine's Remote Desktop service?