You should see two radio devices, wlan1 (2.4GHz) and wlan2 (5GHz), they will only show the supported Band, looks like you’re setting wlan1, or that QuickSet only sets it (or will set wlan2 with same SSID and parameters as 2.4GHz wlan1, haven’t used QuickSet in a while).
Look into Wireless > Interfaces after you finish QuickSet. wlan1 and wlan2 should be there. If wlan2 is not set up, all you have to do is:
Enable wlan2, Mode: ap-bridge, enable WMM, set same SSID as in wlan1, set country, installation (indoors/outdoors) and Wireless parameters (you’ll see is 5GHz).
Point its security-profile to the same one as wlan1
Bridge > Ports Add wlan2 to bridge-local if not already there.
Done.
Devices will connect via 2.4 or 5GHz depending on best coverage transparently.