Can't find out whether your modem supports UPnP, so you could try port forwarding.
Look for Zyxel in the list of routers, there are a few 645 models to choose from.
Choose LW from the list of programs.
Don't forget to set up a static IP first (you'll find a link to static IP setup within the port forwarding guide).

http://www.portforward.com/english/r...outerindex.htm

Let us know if you have any trouble