Ubiquiti Device Discovery Tool Method
If your device is already adopted by another UniFi controller, you should do a site migration
There are also several other options for device adoption
1. Download, install, and run: Ubiquiti Device Discovery Tool
2. Click "Scan"
3. Click "UniFi Family" to switch to the UniFi device view
4. Click "Action"
5. Change the "Inform URL" to your server's set-inform URL
Make sure to use http:// and :8080/inform, which is the protocol and port used for device communication. Always use a DNS name instead of an IP address, that way devices will not disconnect if the IP address of the controller ever changes. Username ubnt and password ubnt are the default credentials.
If the device has been adopted previously, it will need to be factory reset by holding the reset ping for 30 seconds
Device credentials can also be retrieved from the previous UniFi controller under Settings > Site > Device Authentication
6. The device will show up for adoption on ALL sites in your UniFi controller, use the sites drop down in the upper right corner to switch to the site you want the device to be adopted in, then click "Adopt"
7. The device status should change quickly from "Pending" to "Adopting", and then "Provisioning"
8. The device will receive its new configuration from UniFi and reboot, then the status will change to "Connected"
9. From there you can upgrade the device firmware, and begin configuring your network under Settings!