since there is no visable reset button on the WAC6552D-S, how do you reset the device to factory def

Hi @CAWorldWiFi,
There's no hardware button on WAC6552D-S to do the factory default. Once it's registered on the NCC and online, it can be reset to default by removing the AP from the site or the organization. Otherwise if the AP is just registered to the site but not online yet, you can reset it to default via ZON utility or apply system-default.conf configuration on the AP web GUI.
Here is the ZON utility download link for your reference.
https://www.zyxel.com/tw/zh/products_services/Zyxel-ONE-Network-Utility-ZON-Utility/