I read Cliff's guide to Ubiquiti UniFi with FreeNAS and Docker and not being able to adopt an AP because the Inform URL was wrong. We have a few devices we fight with this all the time for so here's the steps to fix.
Sometimes the Ubiquiti UniFi controller has devices that'll show up in the Unifi but don't have the box for changing the inform URL.
The workaround is to SSH into the device after resetting it. The default username and pass are usually ubnt and ubnt.
From here, you can issue two commands and get the device to show up.
Chance the 10.0.1.2 to the IP of your controller.Code:set-inform http://10.0.1.2:8080/inform
The hiccup is that sometimes, this doesn't work. If you run into that issue, you can issue the command again and usually that'll get you to be able to adopt. One Ubiquiti caused me to do this with 2 retries but then it worked.
I like UniFi but it's maddening at times when there's one device that you adopt with the Inform URL box but then there's another where there's no Inform URL box.