I assume there's a manual that can tell you this, a forum dedicated to Westell modems, and/or a website that the manufacturer might have a user's guide for download...
But generally, you have to access any firewall settings a modem might have. Though, to be honest, I've never heard of a modem needing to have it's ports opened/closed. That is generally a job held by a router.
Now I've not seen everything so I'm sure it's possible that you have a modem with this capability biult in, OR you have an all-in-one modem/router unit.
Anyway, with routers, you usually go to the default IP address - 192.168.0.1 or 192.168.1.1 inside Internet Explorer/Firefox, and once there you should be able to access a lot of settings for the router. In here you should have port forwarding options, which should allow you to dynamically open/close ports.
The same might hold true for your modem, but I really don't know anything about them, just giving you a place to start.
Try the above and see if that gets you anywhere. If it doesn't, google a while. Those are your best bets.