Had a problem with 5GHz. on my rt-n66u, too. Went back to 270.26b, and it works fine. Even had a nice saved settings for that firmware version.
These routers are virtually unbrickable *smile*. If the built-in firmware upgrade facility in the web GUI admin page doesn't work to upgrade firmware, you can use the Asus Rescue utility (get this from Asus download page, under utilities) to do your firmware upgrade. To use this utility, get your router into "recovery" mode by turning on the router power with the reset button pressed in, and hold the reset button after turning on the power until the power light blinks slowly...then your router is in "recovery" mode, and you can use the rescue utility.
Also remember to clear your nvram (which removes all your settings) and re-enter your settings after upgrading if you were having problems with the previous firmware (unless you already have a saved settings cfg file that was made when the router was working well with that firmware version *smile*, you can use that).
To clear your nvram, you can do one of these:
1. Press the WPS button, power up the router, and hold the WPS button for 10 seconds or so after powering the router up.
2. Log into the router using telnet at the router's admin address (192.168.1.1 by default), and issue this command:
mtd-erase -d nvram
then reboot the router and your settings will be cleared. There are other ways to clear the nvram as well, but these are the ones that I use *smile*.
It really is pretty easy stuff, and it is very hard to brick one of these routers.