Just wanted to confirm that the 2.4 ghz and 5 ghz have different/unique SSIDs on your EA4500? If not, make sure they have different/unique SSIDs.
Change the 2.4 ghz channel width on the EA4500 to "20 MHz only"
Then go to the link below and download/use the wifi analytic tool (inSSIDer). Try to find the best channel for your 2.4 ghz.
http://www.ampedwireless.com/wifianalytics/
Also test to see if you're having the problem on 2.4 ghz or 5 ghz or both.
It's possible that you have too much interference on 2.4 ghz.
Make sure you reset the network settings on the iPhones. Settings,general,reset,reset network settings.
Change the 2.4 ghz channel width on the EA4500 to "20 MHz only"
Then go to the link below and download/use the wifi analytic tool (inSSIDer). Try to find the best channel for your 2.4 ghz.
http://www.ampedwireless.com/wifianalytics/
Also test to see if you're having the problem on 2.4 ghz or 5 ghz or both.
It's possible that you have too much interference on 2.4 ghz.
Make sure you reset the network settings on the iPhones. Settings,general,reset,reset network settings.