Problem with RDC and Linksys WKPC54G

Hi, I've been going crazy trying to figure this out......

I have 2 computers behind a Linksys WKPC54G router.
Computer # 1 is set up for an RDC.
When I use computer # 2 to connect to computer # 1 - I have no problems.
I can use either the local computer name (mydesktop) or I can use the ip.
address (207.xxx.xxx.xxx) - both will work and establish the connection.

However, The moment I try to use a computer outside the network, ie. outside
the router, I cannot get a connection...... I always get the same error :

The client could not connect to the remote computer. Remote connections
might not be enabled or the computer might be too busy to accept new
connections. it is also possible that network problems are preventing your
connection.

Why is it that i can use the ip address to connect from computer #2 , but i
cannot use the ip address to connect from a computer when i'm away ??
I presume it has something to do with the router settings, but do know which
ones ??

Thanks a bunch.....