I'm using the Asus RT-AC68U but I'm sure it's the same process. I just have two different names and passwords on each band, 2.4 and 5 Ghz. I made sure my rMBP didn't have any networks remembered and only put in the password for the 5 Ghz network. Since this is the only device with AC capabilities in my house it is the only one logged in on that band. So at worst it would be on N but every time I check it's connected on AC. Make sure nothing else is connected on that band. Wifi can only go as fast as the slowest device connected. So it you have a wireless G device connected the computer will never get AC. Now this is only per channel. Having it connected to the 2.4 Ghz band won't effect it but double check you have nothing else on the 5 Ghz band.
Victor