If by any chance the AP still doesn't show at all in the SZ it may be a certificate problem, because that's an old AP. If that's the case you could import a file from the ZoneFlex AP to replace the cert, then upload it in the support site, they (TAC support) send you a file with a new cert to download it again to the AP, that way the SZ will accept it.
Or you could tell the controller to not check certs by CLI, that way the SZ will also accept the AP.
conf
no ap-cert-check
Best regards