This is the procedure to recover to the original firmware after unsuccessful firmware update (courtesy of dd-wrt.com)

Preparation:

1) On a Microsoft windows computer download and install the ASUS recovery tool from:http://www.asus.com/Networking/RTAC68U/HelpDesk_Download/

2) Download the original Asus stock firmware from: http://www.asus.com/Networking/RTAC68U/HelpDesk_Download/

3) Set your windows network settings to a static IP of 192.168.1.5, and subnet 255.255.255.0 (Gateway and DNS not needed).

Recovery Mode Flashing Instructions:

1) Power off the unit.

2) Press and hold on to the WPS button.

3) While holding down the WPS button, power on the unit.

4) Keep holding the WPS button until the power led starts to blink a few times.

5) When the power led starts blinking / flashing, release the WPS button.

6) Power off the unit.

7) Press and hold on to the reset button.

8) While holding down the reset button, power on the unit.

9) Keep holding the reset button until the power led starts to blink on and off repeatedly. (This means the unit is now in emergency recovery mode)

10) Plug in your computer with the ASUS recovery tool directly into Lan port 1 on the router.

11) Run the ASUS Recovery software, and select the stock Asus firmware.

12) Click the “Upload” button.

13) The ASUSrecovery software should detect the router, and start to upload the firmware.

• If the ASUS recovery software doesn’t detect the router, try disabling your windows firewall, and also disabling and then re-enabling your network adapter in windows networking settings.

14) Once the router has flashed and rebooted with the stock firmware, set your network adapter settings back to DHCP. Your computer should pick up an address from the router, and then you can navigate to 192.168.1.1 in the web browser to start re-configuring the router.