Can access computers only when on same network

Can access computers only when on same network

I hope this question has not already been asked. I looked but couldn't find an answer.

I'm using:
Win 7 pro
Droid X 2.2
Windows Companion 2.0.3

I'm using RDP. My Home computer is connected to a Router. I can only remotely access it using the IP address given to the computer by the router when my Droid X is connected to the same router. When I try to access it through 3G or from a different WiFi connection it won't let me connect (Could not connect to server). The same thing is happening with my office computer. Are there any settings in the Router I need to change to make it work?

Chris