I decided to go back to stock firmware for my access points from openwrt but could not boot into recovery mode. The LED started flushing as described in the docs but it seemed to restart the bootloader over and over again, thus not IP address was assigned and I couldn't flash the firmware using tftp.
Yet openwrt was still running, so I extracted the mtd partitions from the stock firmware using (https://github.com/oetken/scripts-ubiquiti-binfile-extractor) and wrote them to the flash directly using mtd utils.
Just wanted to share this if anyone stumbles over the same issue.