The following instructions assumes you already have your internet set up and working with CenturyLink. This is not for CenturyLink fiber setups. The modem I'm using is the ZyXEL C3000Z which was provided by CenturyLink.
To start, connect the dsl port on the Modem/router CenturyLink device (will call it "modem") to the wall phone jack and then plug in the power cable. Also connect the Google WiFi to power. Do not connect the two devices via ethernet yet.
Once they are both powered on, factory reset both devices. I've found this will cut down the number of problems during setup.
Reset Google Wifi: If you've previously set up a network with this device you'll need to reset the device through the Google WiFi app to get it to be removed from your list of owned networks. To do that go to the settings page (gears icon on right side), select "Network & General", select "Wifi point", and click "Factory reset." You can reset only the device by pressing and holding the button on the back the device until the lights turn completely off.
Reset CenturyLink Modem: Press the recessed reset button on the back of the modem using a toothpick or something that fits in the hole. Hold until the power button turns red.
After the modem is reset, wait until it power cycles and then is connected to internet again (as indicated by a glowing green internet status light).
Connect a laptop or computer to the modem using the second ethernet port on the back of the modem. Once connected, open a web browser and navigate to http://192.168.0.1 (or the GUI address listed on the modem sticker). Log in using the modem username and password (also found on sticker).
Go to "Advanced Setup" and then click on "WAN Settings"
Make a note of the PPP username shown on this page. Mine looked like spe*******@centurylink.net
At this point you'll need to contact CenturyLink support via chat and as them for your PPP password. Feel free to use the question I sent to the agent in the chat session. After verifying my identity and account she very quickly responded with my PPP password.
I was wondering if you could provide to me my PPP password for my account? I have the username already "spe*******@centurylink.net" and I'm going to be bridging my modem/router to use a different router I own.
She also mentioned that when setting up the PPPoE settings on the router I might have to use "@qwest.net" instead of the "@centurylink.net" I in the modem config pages.
Armed with your PPP username and password, you can finish "bridging" your modem. Back on the "WAN Settings" page in the "ISP Protocol" dropdown, select "Transparent Bridging." You can also just leave the "Transport Mode" dropdown as "Tagged-201."
Click "Apply." Once the page refreshes you should now no longer be able to access the internet from the computer connected to the modem.
Go into the Google WiFi app and select "Set up WiFi point" and then "Create a new Network." It will start searching for your Google WiFi device and will prompt you to scan a QR code once it's found it.
As you continue with the process it should open a screen asking for your PPPoE info. Type in your PPP username using "@qwest.net" for the ending and the password you got from CenturyLink support.
If all goes correctly, you should then be asked to name your network and choose your password. Once that's done you should be up and running!
@thebenkahn Were you able to resolve connecting C4000XG to the Google Router? I am in the same situation and have tried all the recommended setup with restarting, bridging, entering PPP user/pass, etc., and nothing seems to work. Any insight you have would be greatly appreciated