Is this issue occurring only on this particular AP? Do the devices connect fine to other APs in another location? I had a similar issue, but with chromebooks, and I couldn't figure out what was going on. What I did was change the radio settings on the SSID from 5Ghz only to Dual band with band steering, and for some reason that seemed to fix the problem. All of a sudden the chromebooks were able to see the SSID and connect, so something you could try on your SSID that is giving you trouble.
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