Question : Using Port 3390 on Remote Desktop Control

I changed an XP machine to use port 3390 for Remote Desktop Control.

I can easily connect within our network to the machine's internal IP address.

But from outside using our external ip address:3390  - I cannot connect.

In the router, I put in 3390 in the port forwarding, using UDP and TCP.

What do you think I missed?

Answer : Using Port 3390 on Remote Desktop Control

In my "humble" opinion, you obviously know how to set this up as you have port 3389 working on one machine. Therefore, I would assume you have done 3390 correctly. The one thing that is different is the registry changes and you have verified that. However, as I mentioned earlier the Windows firewall (or any other such as Symantec, ZoneAlarm, McAfee, etc.) would have to be configured manually for port 3390. If you haven't done so already switch it off, for testing, or add the new port exception. To do so open he firewall; control panel | Windows firewall | Exceptions | Add a port
Make sure you you also click change scope and select "any computer (including those on the Internet)"