https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=f0445746f6fd96fc7c5394b238153bd2ff22bc5b
EDIT: as noted by @williamthrilliam in Zyxel NWA50AX Pro bricked, these instructions (which was written when bringing support to the device) will in 50% of cases cause the device to brick itself if OpenWrt installs itself in the incorrect slot, which is unknown at time of flashing when following the above instructions, so EXTREME CAUTION is advised and be prepared to unbrick it. See linked thread (Zyxel NWA50AX Pro bricked - #14 by williamthrilliam) how to unbrick the device if this happens.