The AP is connected over PoE. When the AP turns on, it runs through its cycle to boot up. When the boot cycle is done it restarts and goes through the boot cycle all over again. It will stay on this loop until someone unplugs it.
What version of vWLAN and firmware do you have for the 2020?
See https://supportcommunity.adtran.com/t5/BSAP-2020/ct-p/enterpriseclass-wifi-bsap-2020-sw
You might have a newer revision model that has a higher vWLAN version requirement.
And this could be a PoE negotiation issue, that limits the wattage needed and causes the reboot.
