Is your ISP AT&T U-Verse?
I and my players are having this exact problem. I (the host) do have AT&T Uverse.
Sadly, the current firmware for the 2-wire modem/router has no way to turn off TCP Flood protection in the hardware firewall. The data requests from the clients on the port 1802 trigger this protection and it blocks the requests going to the host machine.
This happens even though the host has the port forwarding on. I have found references in the AT&T forums that this problem happens even if you turn on DMZ to the host machine (which should drop all the router firewall protections).
At present, it looks like you will have to look at VPN or an SSH tunnel...
- Robert