I found to support iOS and hundreds of other mobiles properly, I had to turn off the pop-up window setting for all browsers.
Click “Popup” in the Captive Portal Gateway page and change the pop-up setting to “None” and save the change. This doesn’t remove the authentication page, but it makes the page friendlier to all browsers, not just Safari on iOS. The user account used is still subject to accounting rules and time limits you specify.
iOS tries to open a web page on the apple.com domain when it connects to a WiFi network to make sure you can see any captive portal pages, including the one for Zeroshell. If the request is intercepted by the portal, iOS opens a browser window showing the authentication page. If you don’t see the authentication page, chances are the request succeeded because of a current portal session, or possibly the portal isn’t working at all.