I found this page based on my searching, and due to the exact same problem as you Mac OS 9000.
I hope you've got it working now.
To get my Snow Leopard Server VPN visible to the Internet besides just the LAN — no matter what sort of ports and protocols I allowed through — didn't seem to work.
Go to Firewall Settings > DMZ Host > and enter the IP address of your server. (I'm assuming -- and for the benefit of others -- you've set up the DHCP IP address for your server to be statically set by the Actiontec router.)