I’ve had the same issue …guess it’s a nvram issue
Just start by uboot … And start by downgrading the firmware . Don’t import previous settings …
Then … flash the the next one … till you get to the latest firmware …it’s exhausting … yet have resolved my troubles with my AR300M-ext
Just a side note … when trying to flash a tester firmware … use the latest stable version … go to root advanced settings … flash the tester firmware and make sure NOT to retain any settings from previous firmware

Good lucky …hop this will resolve your issue