Hi Janine,
I am unsure of what kind of router you use. But I was able to use Cisco IOS, ipchains + port forward or iptables. What I did was to have the router listen to 6000 then foward this to my PC on the internal network. Then on the remote server tell that your X server is the router.
Or better yet use SSH tunneling. See -L or -R options. Or just edit the sshd config of the remote server to allow X11 forwarding.
