Your description of your problem is not totally clear, but here is how the NAT should look for RDP:
If Proto Ext. port range NAT IP Int. port range
WAN TCP 3389 (MS RDP) Internal PC 3389 (MS RDP)
The firewall rule will then look something like this:
Proto Source Src Port Destination IP Dst Port
TCP Any * Internal PC 3389 (MS RDP)
If you chance the NAT Internal Port Range, you have to modify the Firewall rule Dst Port to reflect the same change. In other words your Int Port Range, NAT IP in your NAT and the DstIP, Dst Port in the firewall rule must always match.
The only other problem that I can see is that the machine running terminal server is listening on 3390 instead of 3389. Do a google search on how to change this back to 3389 if you need to.