UniFi device adoption by a UniFi Controller can be problematic in certain scenarios and network configurations. UniFi devices attempt to automatically inform a UniFi Controller of their existence through the use of a specific DNS host name or DHCP vendor option.
UniFi devices use a default controller inform URL of http://unifi:8080/inform. If a UniFi device cannot contact this inform URL, then the adoption process will fail.
This post describes the options, and how to configure your environment to get UniFi Controller adoption working reliably.