I had a really strange situation where i was unable to adopt a unifi switch that had been offline for some time. When i tried to SSH to the device,

I found that I had to use the following command to get into the device via SSH.
ssh [email protected] -o HostKeyAlgorithms=+ssh-rsa
Once i added the -o HostKeyAlgorithms i was able to use SSH like normal.

Then when I was attempting to find the firmware files on Ubiquiti’s site, i found that they have hidden the page. After a bit of searching on Reddit, i found the link. It’s here https://www.ui.com/download/releases/firmware

I proceeded to go through numerous updates from 4.0.66 to 4.3.22 to 5.43.23. Once i got there, i attempted to re-adopt the device, and bam, everything started working like normal!