If you have a device that you've tried to set-inform from SSH, but it's not showing up for adoption in your UniFi controller, a common problem is that the device's firmware needs to be manually upgraded before it can be adopted.
There are several ways to upgrade the device firmware locally, but the easiest way is to use the "upgrade http://<link to latest firmware>" SSH command.
Find the firmware
First, you'll need to locate the appropriate firmware for your specific device model on Ubiquiti's downloads page: https://www.ui.com/download/.
In this example, we'll be upgrading the firmware on a UAP AC Lite. If you are unsure what model you have, you can look at the back of the device or issue the "info" command in SSH.
We found the firmware download page for our UAP-AC-Lite here: https://www.ui.com/download/unifi/default/default/unifi-firmware-6018-uap-ac-litelrpromm-proiw.
Under Firmware, click the download icon in the right corner of the top row to download the latest firmware version.
Then click the "I Accept" button to accept the EULA.
Click the "Copy URL" button. No need to "Download File". We only need the link for the upgrade.
Now from the SSH prompt, issue the upgrade command. The exact command will vary by model and firmware version. For the UAP AC Lite in our example, it is:
The device should reboot with the new firmware version which you can verify with the info command. Then you should be able to set-inform from there, or set-default then set-inform if you have any problems.