More information: UniFi - Login with SSH (Advanced)

The automatic discovery for new devices (switches, access points) only works in the same subnet. If the controller is located in another subnet, manual actions are required.

First, log in to the new device via SSH. If it was never adopted to a controller, the default credentials can be used. Then the inform URL have to be set.

Enter the following command. Replace unifi.int.example.org with your controller FQDN or IP address.

set-inform http://unifi.int.example.org:8080/inform

Example:

user@laptop:~$ ssh ubnt@switch
The authenticity of host 'switch (192.168.1.123)' can't be established.
RSA key fingerprint is SHA256:sWpO+/IBxGL4lmxWM4B.o64aN0soYR2M/wrvZ90d5PnX
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added 'switch,192.168.1.123' (RSA) to the list of known hosts.
ubnt@switch's password: 


BusyBox v1.25.1 () built-in shell (ash)


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 |   |   |____ |__\_  ____/__|
 |   |   /    \|  ||  __) |  |   (c) 2010-2022
 |   |  |   |  \  ||  \   |  |   Ubiquiti Inc.
 |______|___|  /__||__/   |__|
            |_/                  https://www.ui.com

      Welcome to UniFi USW-Lite-8-PoE!

********************************* NOTICE **********************************
* By logging in to, accessing, or using any Ubiquiti product, you are     *
* signifying that you have read our Terms of Service (ToS) and End User   *
* License Agreement (EULA), understand their terms, and agree to be       *
* fully bound to them. The use of SSH (Secure Shell) can potentially      *
* harm Ubiquiti devices and result in lost access to them and their data. *
* By proceeding, you acknowledge that the use of SSH to modify device(s)  *
* outside of their normal operational scope, or in any manner             *
* inconsistent with the ToS or EULA, will permanently and irrevocably     *
* void any applicable warranty.                                           *
***************************************************************************

USW-Lite-8-PoE-UM.6.3.12# set-inform http://unifi.int.example.org:8080/inform

Adoption request sent to 'http://unifi.int.example.org:8080/inform'.  Use the controller to complete the adopt process.
 
USW-Lite-8-PoE-UM.6.3.12# 

More information: UniFi Network - Layer 3 Adoption