Hi
If you only use the routing feature of the router there is no access control, and you should be able to connec to the RDP server.
But if you are using NAT as well take a look at this article on port forwarding on IOS Routers: http://www.cisco.com/c/en/us/support/docs/long-reach-ethernet-lre-digital-subscriber-line-xdsl/asymmetric-digital-subscriber-line-adsl/12905-827spat.html
I would however not advise you to allow RDP from all of the internet to you RDP server, but only allow it from trusted internet sources using ACL or similar.