Lion Server - Can access web server, not VPN.

Hello!

I have a 10.7 Server serving both VPN and a website. It has an external IP via NAT in the firewall, and I can access the website perfectly fine via the external IP, however I cannot connect to VPN externally. I have been able to connect within the LAN utilizing the local IP, but whenever I to utilize the external IP both within my network, and elsewhere (via iPhone on cell network) I get the message:

"The L2TP-VPN server did not respond. Try reconnecting. If the problem continues, verify your settings and contact your Administrator."

I'm not really sure what to do at this point. Help!

Thanks in advance!