That's weird... maybe, your computer got infected with spyware and for that reason is blocked the access to windows firewall panel. Try to scan & clean with any anti-spyware tool (like Spybot or...

I didn't found your modem model at www.portforward.com. That site includes some guidelines to open/forward ports to your computer. I think this modem is working as bridge modem only, so your computer...

bad network cable