All the O.S. use the same strategy to find out if they are behind a captive portal. Usually they try to connect to well-known internet URL, of which they know the content.
If you are using level three access you will need to add the following into the walled garden . This will disable the CNA window on apple devices which is needed to join the email confirmation session .
www.appleiphonecell.com
•*.apple.com
•www.itools.info
•www.ibook.info
•www.airport.us
•www.thinkdifferent.us
•*.apple.com.edgekey.net
•*.akamaiedge.net
•*.akamaitechnologies.com