The Zyxel C3000Z DSL modem/router (at least one I lease from CenturyLink) is able to be placed in transparent bridging mode by going to Advanced => WAN settings => ISP Protocol and selecting Transparent Bridging. This turns off the router and wireless functionality and makes it function like “just a modem”. After doing this, I can use my own OpenWRT-based router/access point to connect with the WAN port set to PPPoE. It’s been quite a stable configuration for a while now, but I just experienced an afternoon of connection problems leading me to want to inspect the modem’s system log.
After enabling transparent bridging on the DSL modem, it will no longer run a DHCP server, meaning that plugging into it via Ethernet will no longer give you instant connectivity to its web-based administration interface. However, it will keep its old IP address and be accessible if you manually set your IP address to be in the same subnet. In my case the modem came set to 192.168.0.1, so I gave myself the address 192.168.0.2 and browsing to http://192.168.0.1 got me to the admin interface. If you don’t know the IP address of the DSL modem, it may be printed on a sticker somewhere on its case.
You can do a similar thing without having to physically plug in a new cable by adding a virtual interface to your own router using the same physical port that you use for the WAN connection. Assign this virtual interface an IP address in the same subnet and you should be able to browse to the C3000Z modem admin page. Just be sure to firewall it correctly.