I'm fighting since over an hour with a device here (tp-link cpe210). I flashed a sysupgrade over luci (from 19.x to 21.02.2) with "keeping settings" enabled.
After upgrade I lost control over the device (was configured as a dumb AP) so I teared it down and entered failsafe directly wired to my client. I put firstboot in the ssh console while in failsafe and then rebooted.
The device showed 21.02.2 and was on it's default setting. So far so good.
But then I configured it be used as a dumb AP but when I power cycled the device it reverted the settings back (even saved & applied successfully before!)
I then did it again, set it all up saved & applied settings with success and used restart in luci back on default! Why?
The flash could be full. That device only has 8 MB and more than usual is set aside for OEM partitions (384K) Are you trying to install any packages?
On the clean firstboot, before changing anything, check the free space under System--Software. If it shows a remarkably large amount (half of the RAM, or about 30 MB), the overlay has been mounted on RAM due to a problem with the flash.
Also now that I flashed with my client directly connected to the device it also rendered the device unusable again (like when it was remotely installed) and I needed to activate failsafe and do a firstboot to "recover" it into a working state.
Yes if the LED doesn't blink in the OpenWrt pattern when booting, it is bricked. You've probably done something like force in a factory image where sysupgrade was called for. The OpenWrt kernel isn't starting at all, so a bootloader based method like TFTP recovery will be necessary.
Lucky me, had it plugged in now for a long time and suddenly it showed the eth0 led and actually works. So now from 19.x again to 21.02.2.
Actually what I did before it magically came back was to keep the reset pin holding for more than 12 seconds when plugging in power. I remember the former owner told to do than before I could set up the devices initially.
due to the change of systems from ar71xx to ath79 thing are not the same as normal for these devices
the next thing I would try is to flash the factory image via your routers recommended recovery method
I had this problem also when going from a 21.02.x to another 21.02.x snapshot, and eventually realised the problem was due to the fact that every new flash alternates between data partitions for settings. So when I went from 19.x to 21.x I took everyone's advice to not keep settings, that was successful.
But when a few weeks after trying another snapshot, I ticked the keep settings, which effectively copies current settings across to other data partition which was still formatted for the old 19 x system, and something about the formatting (compression from memory) is not compiled into the new kernel (as seen with dmesg looking at in early stages of mounting data overlay partition), so it cannot mount that partition any more.
The solution was to flip back to the previously loaded snapshot and take a backup and choose option not to keep settings which will do a format of the new data partition, or work out which mtd is the second partition and kill the formatting on it so it can be rebuilt properly on first boot after the flash (Linksys WRT1900ACSv2 fyi).