Zywall USG 20 and the Liftmaster 828LM Gateway

I'm trying to get a LiftMaster 828LM Internet Gateway working with my Zyxel 20, and for some reason it just repeats the DHCP cycle over and over - request IP, assign IP, confirm IP, wash, rinse, repeat. The Chamberlin support tech suggested that I increase UDP timeout to 180 seconds, which I have done, and make sure the firewall isn't blocking UDP Port 80. I fixed the timeout via the CLI, but I am not sure what needs to be changed to assure that UDP 80 is open through the firewall. Has anyone used that Liftmaster gateway? Any thoughts on that UPD port 80? Thanks!