Should have mentioned a few other things...
I'm on Windows 8.1 x64
I've tried fiddling with settings in my router and the adapter. It will connect to the 5ghz band when that band is enabled in mixed mode OR 802.11a mode. Again, I think this all relates to the adapter settings which offer every option (a/b/g) EXCEPT n.
I've tried uninstalling/reinstalling drivers
You may want to have the encryption set to WPA2-AES, avoid any option related to TKIP and use channel bonding (40 MHz) as per articles below.
Here's the solution that will most likely work.
You need to enable WMM on the router's QOS settings.
The reason is that compliant IEEE 802.11n WiFi adapters must have WMM enabled to get the N speed (GHz band doesn't matter). Non compliant ones like your Atheros will still run on N speeds despite the WMM being disabled.
Check out this link for more info: Don't Mess With WMM! - SmallNetBuilder