W3400V is a wireless modem. First of all, your PC must have a static IP.
1. Open a web browser, type in address "192.168.1.1" and press Enter.
2. Key in the username and password for the modem and click Login.
5. Choose custom service and type in a name/identifier so that you know why and what this open port is for.
8. Reboot your modem/router for this to start working.