Adopting Unifi AP to Controller on Different Network or Subnet
Unifi controller will search and automatically detect Unifi AP on the same subnet by default. The controller will not find AP which located in different subnet. For example, the AP installed on 192.168.100.0/24 subnet and the controller installed on 192.168.10.0/24 subnet. I assumed the IP address of the AP is 192.168.100.254, 192.168.10.10 for the controller and the network already routed as Access point and Controller can ping each other.
The first thing you should do is remote the access point with SSH. You can use PuTTY or XShell as your ssh client. Login with default credential (username: ubnt, password: ubnt). If you cannot logged in, you should hard reset the AP.
After you logged in, issue this command
The access point will appear on your controller after you issue that command and ready for adopt. Adopt your access point by clicking the adopt button and then issue set-inform command once again via SSH.
Your SSH will automatically disconnected after access point adopted on the controller.