Some iOS devices are not redirected to the Captive Portal when the user gets connected to WiFi network with iOS device. 

The Captive Portal is not triggered automatically when is the testing domain accessible. If the testing domain is not accessible, CNA (Captive Network Assistant) popup is shown.

How iOS determine if it has an Internet Connection

  1.  The iOS and Mac OS X devices check for the following domains which are hosted on Akamai network:
    and other domains used for this purpose by Apple
  2.  If the domain is accessible and returns “Success“, the Captive Portal is not triggered automatically. “Success” response means the device is connected to the internet.