Ahh right ok. All you're doing there is opening a port.
It isn't clear from the screen shot if your router can do port redirections (I.e. if you RDP to port 3390, it would redirect it to port 3389 on the LAN PC).
The following link shows how you can change the RDP listening port on a PC. If you change the listening port to 3390 it should work: How to change the listening port for Remote Desktop
It mentions changing registry settings so take a backup first.