@FutureMagic 

If you had Smart Connect on, then the single 2.4Ghz SSID and P/W was used. With it off, and you had never set the 5Ghz SSID and P/W it was using the shipped dafault.

On some devices, they will connect to the faster or stronger SSID (or try to). At distance, the 5Ghz will be stonger than the 2.4Ghz SSID, although the 2.4Ghz SSID does go further. So I suspect the reason was those devices were trying to connect to the 5Ghz using the 2.4Ghz credentials.

You should have seen that though when you looked at the devices that couldn't connect and tried re-connecting them? You should have seen BOTH SSId's? That should have clued you in you had Smart Connect OFF. By the way, in some cases, depending on device, if it supports 5Ghz, you should use it, you might get a faster connection depending on some factors, one being the ISP speed, over 100Mbps it usually will be faster on 5Ghz than 2.4Ghz.

Glad you are working though.