As per release notes for WiNG 22.214.171.124-034R (Public) there downgrade limitation to a previous release.
These restrictions apply to all primary and derivative access points. i.e. AP7522 and AP7522E
Running from partition /dev/mtdblock6
Validating image file header
Update error: Aborted, WING EXPRESS Controller cannot be upgraded to WING
This restriction is imposed by the FCC part 15.407 rules effective June 2, 2016
APs sold before June 2, 2016 can be downgraded.