I'm thinking that your problem is more in the modem config, rather than in the Firebox config.
Reason: that modem is actually a modem/router.
The best (read: most reliable) course of action is to configure the modem to operate in Bridge mode. This will reduce it to a simple modem, and allow your firewall appliance to actually do its job unimpeded.
Instructions:
http://www.dslreports.com/faq/13600The only problem that I see, after looking thru the product specs for your appliance, is that it lacks in a DHCP server. Since putting the modem into Bridge mode will also disable its DHCP server, you'll need to set up something else on your LAN to handle that duty.
Fortunately, there's an add-on for WHS to do that for you:
DHCP4WHSHTH,
Chris
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