Sorry about that, I didn't recall that this feature wasn't in the original Asus firmware. I've been using Merlin's so long I just forgot.
You can flash Merlin's firmware quite easily and if you don't like it, you can revert back to your stock Asus firmware, as long as you make sure the version you are running is still available for download at the
Asus support website for the RT-AC66U.
Flashing your firmware is really easy. Just download the version you want from Merlin's site
http://asuswrt.lostrealm.ca/ (read about the different features here first to see what you want to use first; I would suggest using the latest stable versoin). Unpack the zip file and put it where you will be able to locate it on your computer. Next go into the router's GUI, and navigate to to the "Administration.-->Firmware Upgrade" page, and find the "New Firmware File" link (browse your computer and find the unpacked file with the .trx file name), and select that to be uploaded to your router. Then click on apply. The router will do the rest.
You should then do a "factory reset" of your settings. This is easily done too. After the upgrade, go back into your GUI, and again on Administration go to "Restore/Save/Upload" and look for "Restore Factory Defaults". Click that and apply. The router will again reboot. You will have to re-enter everything by hand again, but I'm guessing that you only have the most simple settings anyway.
Once you have Merlin's firmware installed, follow the directions in my first post above.
Good luck.