Digi USB 2 port behind firewall

I am using the Digi USB 2 port behind a firewall. I have done the port forwarding (3422) at the router. But I still can not get it to connect to the Digi USB 2 port? I use the correct public IP-adress for the router. I am using the Configuration manager and the Discovery List but it can not find the Digi USB device. What else is to be done?
Does anuone know I can distinguish between the two USB ports at PC Host (when I get it working). Different port numbers or what?
I run Win 7.