Your best bet is probably, like mentioned above, to assign a different port and do port-based dnat (port forwarding) to your internal servers based on their ports.
Map 3389 to your Internal server (192.168.0.5)
3390 to another machine (192.168.0.6)
3391 to another machine ... etc..
Then, using MSTSC, you can specify an alternate port by using the WAN_IP:port syntax (188.8.131.52:3390)
But it would be considered a better practice to open these ports through a VPN (PPTP works well) or at the very least, limit access to a given source IP address.