|You have to forward the port 3389 to the computer, you want to allow Remote Desktop to.|
Therefore, go into the routers setup / security / rules.
In the Inbound Services, you have to add a new rule, so click the add button.
Under Services, choose Remote Desktop (RDP) if exist. If this does not exist, you first have to create a new service for that.
If you have to create that service, go to the Services, click the Add Custom Service, give it a name, e.g. Remote Desktop, type TCP, Start Port: 3389, End Port 3389 and click apply.
Then go back to Rules and add a new Inbound Service.
From the drop down list, choose your Remote Desktop service.
Action Allow always, Send to LAN Server: the ip address of the computer you want to allow incoming Remote Desktop to, WAN Users: any. Click apply.
Try to connect via Remote desktop.