The 5GHz wireless radio is likely disabled, either manually or by a settings reset. It is not necessarily broken.
First, check the LED on/off button. Press the LED button on the front of the router. This toggles all LEDs, including the 5GHz LED. Press it to see if the 5GHz light turns on.
If the LED stays off, log into the router's web interface at http://router.asus.com. Navigate to the Wireless settings. Verify the 5GHz network is enabled and broadcasting an SSID. The default 5GHz SSID is separate from the 2.4GHz one.
IF THAT DIDN'T WORK: Perform a power cycle. Unplug the router's power adapter for 30 seconds, then plug it back in. If the 5GHz LED remains off after a full boot and the radio is confirmed enabled in settings, there may be a hardware fault.
HEADS UP: Some devices only connect to 2.4GHz networks. Ensure your client device (phone, laptop) supports 5GHz Wi-Fi.