@5spdvl
Every device that is connected to network has IP. In your case the local desktop is in the LAN and has false IP designated by the router(e.g. 192.168.1.xxx or 10.10.10.yyy). To use remote desktop to it you can either do ssh tunnel with port forwarding to a *nix machine on your LAN, or you can set up the router to map its outside port xxxx to port 3389 of the desktop you are trying to access.