With pinggy.io you can reach the desktop of a Raspberry Pi from anywhere with just a single command. Replace the port 3389 with the port where your remote desktop server is listening.
Install an RDP server on the Pi first, then run the tunnel command on the Pi itself:
This is a TCP tunnel, so Pinggy gives you a host and port such as tcp://example.a.pinggy.link:11223 rather than an HTTPS URL.
In your Remote Desktop client, enter the host and port as example.a.pinggy.link:11223, without the tcp:// prefix.