Unable to Set up Co-op games over internet :( Help please!

[sgtredphoenix 6]

Hey, I'm trying to set up a CO-OP server for this mod;

Coop mission : Dynamic Zombie Sandbox by bobtom updated

When ever I set up a server or my friends do we can never find each others servers. I've set up Port forwarding for the default port 2302, I have also tried 9987 which I use for teamspeak server on my machine. The server appears for me on both internet and LAN but not my friends.

I have even tried Hamachi but the game never shows. We have all tried LAN and internet, searching by name and I.P address can anyone help?