For many weekends (5?) over the past couple years I've spent hours trying to get my Zyxel NBG-419N to work in "Router Mode" instead of "Access Point Mode" when operating behind our (old) DSL modem, the Actiontec M1000. From juggling DHCP settings, to NAT-enabled settings, to Transparent Bridging, to attempting to configure PPPoE authentication and DNS servers by hand, I had a lot of trouble getting the Zyxel and Actiontec to play nicely together. The end goal was to have the router in "Router Mode" and authenticating with the ISP directly, so port forwarding and UPnP would work better (plus I like the Zyxel interface more). Eventually I figured it out though, and the solution was ultimately a lot easier than I was expecting.