While trying to get my Asus RT-N12 router to work in Repeater mode, I spent quite a lot of time and stumbled upon some methods that seem to work for me (and my version of router). Particular thanks goes to information found at the following two links:
I also found that the "Wifi Analyzer" app (for my Android phone) was quite helpful in determining the functionality of my system during the many attempts at success.
Here's How to get your Asus RT-N12 into Repeater mode
First, verify that you can communicate with the Router:
Adjust your PCs TCP/IP settings to allow communication with
the default settings of the Router (192.168.1.1). I changed my settings
Connect the PC cable to one of the LAN ports to see if you get a response from the router, after it has powered up and the Radio (2.4GHz) LED is blinking. When I was most of the way through the process, with the system repeating one of my Wi-Fi signals, I was unable to re-establish communication over the Ethernet link into the LAN ports. I found that resetting the NVRAM to its default values allowed me to communicate, again, so I could select an alternate Wi-Fi signal to repeat.
Here is the whole process, starting with the resetting of the NVRAM:
Set the Mode Selector switch to the Repeater (middle) position.
Depress the Reset button while applying power, holding the Reset button down for another second or two after power is applied before releasing it. After a few seconds, the Power LED should start slow blinking at about a 5 second cycle rate.
On the PC, Browse to 192.168.1.1
Click the "Restore default NVRAM values, which should open another page indicating "Command nvram erase completed."
Clicking "Continue" should return you to the previous page.
Click "Reboot" and wait for the Radio LED to begin blinking again (about 30 seconds).
Browse to 192.168.1.1, again, and you should find the main page showing your default SSID (mine is "ure"). If you check with Wifi Analyzer, you should see the "ure" SSID on a strong signal (if you are near the RT-N12).
Selecting the top icon in the center of the display (text: Disconnected) should change the right portion of the display to "Access Point status", with a button marked "Go" to the right of "AP Survey". Clicking this Go button should open the page 18.104.22.168/survey.asp, which should display all of the Wi-Fi signals that are being received. Select the signal you want to repeat, and then click the "Connect" button. You will be prompted to "enter Network Key". After entering the key for the signal you want to repeat, click the "connect" button. The RT-N12 should configure itself to repeat this signal, and then perform a reset, blinking the lights. If you are watching the Wifi Analyzer, the "ure" signal should disappear, being replaced by the selected signal within about 30 seconds.
If you find better/easier/faster ways to perform this configuration, please let me know and I can update this page.