If you have multiple PC's you want to connect to via Remote Desktop you
have to change the TCP port remote Desktop listens for each additional PC.
Then, you'll have to create rules in your firewall that coincide to the
TCP port each PC is listening on. When you connect from the Public
internet you'll include the TCP port in your Remtoe Desktop connection.
For example, suppose your WAN IP is 68.12.13.44 and you have two PC's to
Remote Desktop into (PC "A" and PC "B"). On PC "A" you leave the default
listening port of 3389 but for PC "B" you change the port to 3390. When
you connect via Remote Desktop from the Internet, your destination for PC
"A" would be '68.12.13.44' but your destination for PC "B" would be
'68.12.13.44:3390'. Of course, you'll have to 1) make sure you have
firewall rules which correspond to the correct PC and 2) make sure you are
using DHCP reservations so the internal PC's always get the same IP
address (or manually configure IP info on the target PC's).
Cheers!