So basically what you're saying is - if the desktop and laptop are both on your network, RDC works fine, but if your laptop is on a different wireless network and tries to connect to the desktop, RDC fails?
If that's the case, it's because port forwarding isn't necessary unless you're connecting from outside the internal network. That's why the laptop is able to connect to the desktop when they're on the same internal network. What you need to do is setup port forwarding in your router configuration - forward TCP port 3389 to the IP address associated with the desktop. Then try connecting to it from a different network.
Thread Moved: Applications -> Networking