Remote Connection over WAN

I am trying to Remote Desktop to my friend’s computer, seemingly having all and correct information, but continually getting the error that the remote system cannot be found.
Both systems are running WinXP (from Home to Pro), through a hub. Both have a LAN Ip, and when connected to the internet, each has a separate WAN Ip. I know you can connect from Home to Pro in this manner (without any further software) as it has been done before. The route of the connection would be “system>through hub>through hi-speed modem>back through modem>through hub>to other system” Am I missing something else?