This article matches the current DD-WRT firmware the closest, and I got things working eventually using a Netgear WGR614v8 and DD-WRT v24-sp2. The firmware UI makes it even easier to set up than the article describes. You just need to:
- Use an ethernet cable to connect to the client router. Connect the admin page (usually http://192.168.1.1) and change the admin username/password as prompted.
- Go to the Wireless tab and select "Client Bridge", and enter the SSID for your existing wireless LAN. Set up the WLAN type and encryption if needed.
- Make sure your WEP/WPA key is entered correctly - the "unhide" option is useful. (I had a typo which initially made things fail)
- Back on the Basic Setup tab, you should see that the WAN interface is disabled, and the DHCP server is disabled too.
- Give the client router a static IP on the same subnet as your main router. (E.g. my main router is 192.168.1.1 and my client router is 192.168.1.2). If needed make sure your primary router is assigning DHCP addresses in a higher range, e.g. starting at 192.168.1.100
- Apply the settings, and refresh the machine that's plugged in to your client router. (Un-plugging and re-plugging the ethernet cable is the easiest)
- You should now get an IP address from your primary router, and be able to ping the primary router's IP, 192.168.1.1 in my case. You should be able to connect to the Internet.