I installed an L2TP VPN network on an iMac using OS X Lion Server app. It works fine in testing when I use my wireless access on the network (accessing the external IP address), but am unable to access from other WiFi networks. What did I miss?
Double check your port forwarding on your firewall/Router. Ports 500, 1701, 1723, and 4500 should all be pointed at your server. All are UDP except 1723 which is TCP. Since the VPN connects from inside the network, the ports forwarding is almost certainly the problem.