I think this may be a problem with firmware signing: https://forums.ruckuswireless.com/conversations/smartcell-access-points/what-is-the-minimum-standalo...
Long story short: download the most recent firmware for both the SD and the APs (standalone) update all manually and try again. For R500 it is possible that you cannot directly update to the most recent version (which should be this one): https://support.ruckuswireless.com/software/2431-zoneflex-solo-access-point-110-0-0-0-2005-ga-softwa...)
In this case you need this FW as intermediate (according to this entry: https://support.ruckuswireless.com/articles/000008727): https://support.ruckuswireless.com/software/1016-zoneflex-r500-indoor-ap-104-0-0-0-1347-ga-software-...
As explained by Lukas, this could be firmware compatibility (image signing compatibility issue).
Please upgrade the controller to highest supported version and then check if APs are connecting or not.