Every router should have a web interface, accessible through 192.168.0.1 (or 1.1) (write the address in the browser)
From there, you should configure Port Forwarding (or only Forwarding) to send all traffic on a certain port (2400 I think is the default for Acknex) to a local computer. In the menu you input the port that a client requests (2400) , the port that the local computer responds on (2400) and then the local computer's IP address.

Then the client should connect to you on the router's IP address (not your local!)

*EDIT* The router's Firewall doesnt matter in this case. The router simply doesnt know who to route the traffic incomming on port 2400... This tells it to route the traffic to your computer.