I've been a long time RDC user and I've never encountered entering dual IP addresses to connect to a computer behind a router.
To access a single computer behind a router;
Configure your router for port forwarding to port 3389
Now in RDC just type the IP address supplied by your ISP
If you have multiple computers behind a router;
You have to configure each computer to listen to different ports to avoid conflicts by changing the registry key in HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp\PortNumber
The suggested ports to choose to avoid conflict with other programs on the computer should be between 49152 and 65535
Now take that number you've chosen your computer to listen to and configure your router to port forward to that number
In RDC, your format will now be ISPIPAddress:PortNumber (ex. 184.108.40.206:1234) to connect to that specific computer behind the router Important:
If you have a firewall software running, that has to be configured to allow communication to the port you opened in your router as well. Note:
The above steps assumes that you're not part of a domain as well as the computer you're trying to connect to.
Hope this helps