If you have not already done so, perform a Factory Default Reset on the 5th Gen AirPort as follows:
Power off the AirPort and wait a minute
Hold in the reset button first and keep holding it for an additional 7-8 seconds while you simultaneously plug the power back into the AirPort
Release the reset button after the 7-8 second hold and allow a full minute for the AirPort to restart to a slow, blinking amber light status
The AirPort is now reset and can be set up again.....assuming that is is working normally. If you try the reset a few times and the settings cannot be changed on the AirPort, then you have a defective AirPort that should be returned to the seller.