Given the internal machines on on the other side of some router and share one IP address, your solution of using the MS Remote Access is going to fail. I will not be offering a reason why or how to possibly fix, but will note that only one machine could be accessed if you forwarded the ports needed. I will not tell you the port numbers since you need to learn how to find these on your own.

What can work is TightVNC since you can assign each machine its own port number and port forward each to each machine. I am not offering any assistance for TightVNC either since it's very well documented and your questions would be answered in the documentation.

Bob