Just some food for thought, 2.4Ghz penetrates further than 5Ghz so if you're walking into WiFi coverage from somewhere else, your device is likely to see the 2.4Ghz network first and connect to that instead of 5Ghz.
In addition, it seems that iOS devices scan 2.4Ghz first before 5Ghz. An easy way to test this is to go into settings and then turn off WiFi. Then turn on Wifi. You'll see all the 2.4Ghz APs pop up first, then a few seconds later the 5Ghz ones will show up. That's why iOS devices tend to favor 2.4Ghz APs over 5Ghz, especially if you have both setup on your device.