I've had plenty of UBNT gear die on a sunny afternoon for no apparently reason. literally dozens of stations of the approx 1k radios we had total.
as far as that problem is consered, the backup bank kicks in on 5 failed boot attempts, you can try and artificially cause a problem by powering it, wait about 10 seconds so its mid boot and cut power, do this 5 times and try to access it. that might cause the backup bank to kick in.
there also was a firmware version that had a DNS snag to delay access to the gui when the radio doesn't get a valid DNS address. i don't recall the version but the radio just needs to sit 15 minutes and you'll get it. then upgrade to the latest version.
if that doesn't work, try taking away your DHCP server from this unit, let it boot and try to access the radio 169.254.1.1 let it sit about 2 minutes to be sure its ready to grab. if the firmware is extremely old, use the reset pin on the radio itself and try to access via the default IP address again. the reset procedures have changed slightly since 1.x verisons