With pinggy.io you can reach the Remote Desktop server on your machine from anywhere with just a single command. Replace the port 3389 with the port where your RDP server is listening.
Turn on Remote Desktop first (on Windows, Settings → System → Remote Desktop), then start the tunnel. 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.