Adopt an AP in Mesh Mode
When you cannot use a hardwired connection with a new AP, you can have it mesh connect to the other APs on site. By default, this feature is enabled at the site level.
To mesh adopt an AP, simply plug the AP in with something like a PoE injector, log into the controller on your phone or laptop and sit tight.
After about 60 seconds, the AP will automatically send the mesh adoption signal out to the other UniFi APs in the area and the AP will be available for adoption in the controller.