If you can SSH into the device, it’s possible to do L3 adoption via CLI command:
1. Make sure the device is running updated firmware. See this guide: UniFi – Changing the Firmware of a UniFi Device.
2. Make sure the device is in the factory default state. If it’s not, run the following command:
sudo syswrapper.sh restore-default
3. SSH into the device and type the following and hit enter, substituting “ip-of-host” with the IP address of the host of the UniFi Network application:
set-inform http://ip-of-host:8080/inform
4. After issuing the set-inform, the UniFi device will show up for adoption in the Devices section of UniFi Network. Once you click Adopt, the device will appear to go offline or have the status of “Adopting” then proceed to “Provision” and “Connected”.