I've restored AirOS to a Ubiquiti device configured for AREDN. Follow Chuck's link to the Ubiquiti webpage and download the desired AirOS file.
Then use the initial AREDN firmware installation instructions for a Ubiquiti device (power on while holding the reset button down until the alternating light pattern starts) and instead of tftp'ing the AREDN firmware, tftp the AirOS file.
To restore Air OS, I believe it does not matter what OS is or is not on a Ubiquiti device.
There should be AirOS recovery instructions at ubnt.com (or maybe OpenWRT?) somewhere.
Does
https://scoop.co.za/blog/ubiquiti-tftp-recovery
https://www.youtube.com/watch?v=o__g7H5YdSw
https://www.ui.com/download/airmax-m/nanostationm
help?
73, Chuck
Then use the initial AREDN firmware installation instructions for a Ubiquiti device (power on while holding the reset button down until the alternating light pattern starts) and instead of tftp'ing the AREDN firmware, tftp the AirOS file.
73s,
Collier
NM7B