Hello,
The settings for Client Autologin only work if you're using the redirection method that Mr. Dvorak has shown.
Client Autologin options do not work unless you're using the "External" method for redirection. If you're using DNS proxy, PBR, or another reauthentication type you'll need to check the NAC allowed domains settings to make sure NAC is not allowing communication out.
Make sure that !captive.apple.com and !gstatic.com are both in this list if you have PBR. They will tell NAC these are not allowed domains and the traffic will be dropped.
iOS will attempt to reach out to at least captive.apple,com, and android will try gstatic.com/generate_204. If they can't reach these sites they detect they are in a captive portal and will prompt the user.
Thanks
-Ryan