I recently got a second router in an attempt to speed up the video streaming from my D2Plug to a PS3. The issue that I originally had with streaming was that since the PS3 was connected via wireless, but the video would start lagging if there was any network traffic. In order to help with that, I got another router so that I would have more ethernet ports to connect to.
However, when I tried to plug the new router into the internet port of the old router, it essentially made two private networks which was going to make using them a pain. Instead, I needed to switch my old router to act as a switch rather than an actual router.
Luckily I found the following post which helped me to set it up properly.
Disable UPnPT Disable DHCP Change the LAN IP address to an available address on your network. The LAN ports on the router cannot accept a DHCP address from your other router. To connect to another router, please follow the steps below: 1. Plug the power into the router. Connect one of your computers to the router (LAN port) using an Ethernet cable. Make sure your IP address on the computer is 192.168.0.xxx (where xxx is between 2 and 254). Please see the Networking Basics section for more information. If you need to change the settings, write down your existing settings before making any changes. In most cases, your computer should be set to receive an IP address automatically in which case you will not have to do anything to your computer. 2. Open a web browser and enter http://192.168.0.1 and press Enter. When the login window appears, set the user name to Admin and leave the password box empty. Click Log In to continue. 3. Click on Advanced and then click Advanced Network. Uncheck the Enable UPnP checkbox. Click Save Settings to continue. 4. Click Setup and then click Network Settings. Uncheck the Enable DHCP Server server checkbox. Click Save Settings to continue. 5. Under Router Settings, enter an available IP address and the subnet mask of your network. Click Save Settings to save your settings. Use this new IP address to access the configuration utility of the router in the future. Close the browser and change your computer's IP settings back to the original values as in Step 1. Connect to Another Router 6. Disconnect the Ethernet cable from the router and reconnect your computer to your network. 7. Connect an Ethernet cable in one of the LAN ports of the router and connect it to your other router. Do not plug anything into the Internet port of the D-Link router. 8. You may now use the other 3 LAN ports to connect other Ethernet devices and computers. To configure your wireless network, open a web browser and enter the IP address you assigned to the router. Refer to the Configuration and Wireless Security sections for more information on setting up your wireless network.