How to configure a Zyxel router behind an Actiontec DSL Modem (Centurylink, Qwest)
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.
- The first step is to get your PPPoE authentication details sorted out. By browsing to your modem on your network (for me, it was 192.168.0.1) you can see your username part. It'll probably be an email address ending in @qwest.net.
- The next step, unfortunately, is to figure out your PPPoE password. I found a guide online to telnet into the modem and extract the password, but this turned out to be just a hashed password (or something), not the original password. So you'll probably have to call Centurylink for this (and answer lots of security questions, including your account from the first step. It should like 8 uppercase letters and numbers.
- Now for the interesting bit. Rather than configure the Zyxel ourselves, we can have it autoconfigure itself. I did a factory reset (you know, using a paperclip) on the modem and the router at roughly the same time (modem first). Next I browsed to the Zyxel, http://192.168.1.1/ and authenticated using the default password "1234". Finally, you'll get to the elusive "eaZy 123" setup wizard, which looks like this.
- Soon you'll be prompted for an Internet Connection Type, which defaults to DHCP. Do not leave it on this setting; change it to PPPoE. Leave the "Dynamic IP" radio option selected and enter the PPPoE username and password you got earlier.
- The rest of the steps (router password, wireless security) should be self-explanatory.
And now you should be set! You may have to release/renew the IP address on your devices (Windows 7) if the subnet changed (mine did from 192.168.0.x to 192.168.1.x, I think). The only potential problem with this solution is now I can't access the modem's web interface...but everything seems to be working so I don't really care 
But previously the modem was 192.168.0.1 and the router was 192.168.0.2, and now the router is 192.168.1.1 and the modem is...hiding.
In recap, the steps are 1) get your credentials, 2) reset your hardware, 3) let Zyxel's internet wizard do its thing.
Hope this helps someone!