Hi
That's a pitty it wasn't as simple as that. It could be faulty of course. The 5GHz is provided by a separate card installed on the main board so that could be faulty leaving 2.4GHz, which is part of the main board, working normally.
Have you tried a factory reset just in case?
The 8324 is only 2.4GHz, with the 8924 being AC so I expect support were getting confused.
If it helps mine is set to 802.11n/ac mixed, I've also set Regulatory mode to 802.11h (this is under the others tab) and channel is set to 40/80 at the moment, although 36/80 works as well. Security is set to WPA2-PSK, Encryption is AES.
Regards
Phil