It does not deploy captive portal on some mobile devices.
Currently I have a firewall that takes care of the captive portal and this works well with some cell phones, but when I try to connect to a neffos he does not run the splash on the cell phone and does not allow navigation, as it is not authenticated.
Depending on the device platform, some end clients donot open the splash page automatically. Did you try and open an HTTP website on a browser on that phone and see if that got redirected to the splash page? cnn.com or espn.com are your best bet.