Windows 7: Remote Desktop & Port 3390

My router does not allow any traffic on 3389 so I need to create some port mappings to access various computers on my LAN via remote desktop. I have the following:

computer1: map 3389 to 3389
computer2: map 3390 to 3389

computer 1 is no issue but when I try RD with computer2:3390 it fails. Is it not possible to map 3390 to 3389 and access the computer without having to change the listening port?

As background information, some of the computers are laptops so they move to different LANs where I also need to access them via Remote Desktop. At the other locations I don't have access to the router and therefore can't change the port mapping (but the router there allows traffic through 3389 so listening on 3389 works just fine).