Port forwarding will not work.

I've setup routers/modems before so this is not my first rodeo.

I've configured the router properly on the 192.168.1.0/24 network and all of our internal clients can access resources outside the private network.

My DynDNS address will successfully ping resulting with the internet IP address assigned to the router.

So, we can access the Internet but external traffic can get reach the router.

Port forwarding has been set up in the router with port 3389 forwarding to the IP address of the server where we want the users to access.

We can successfully internally ping the IP address of the server which accepts port 3389.

The problem is, that the router will not fulfill the forwarding of the external traffic to the internal server.

The router is a Cisco device type BWG Model# DPC3941B

Calling tech support has so far resulting in no success.