Is this AP managed by a controller or you are using it in standalone mode?
Please confirm the firmware version.
If it is running on stand alone mode, make sure you have configured both the radios as 2.4G and 5G radio setting are different in standalone mode.
If you think configuration is correct and still 5G radio is not working, SSH into the AP and run below command.
This will help us to know if WLAN is created and is showing up for 5G radio or not.