How to spawn "sign-in to network" page for a captive portal?
It depends upon the firewalls vendor and how they implement it. If you're using a Cisco, such as one of the Meraki devices- the captive portal spawns best under Firefox versus Chrome/ium or Opera. And also another way to catch it, is to


Disconnect from the access point 
Clear any saved credentials
Re-log in


Take care to notice the notifications you're likely to get a prompt. Follow the prompts. If your AP is as I have mine set up, it is likely a click through walled garden, these pages include a check box radio form entry field, You need to fill the check, then click 'continue' .

Also, iOS has to always be the Marsha Brady of the tech bunch, be sure that in the firewall settings of the AP, you have the correct ports available for iOS to get through otherwise if you're on and iPhone or pad, you won't get the chance to sign in, it will just auto-deny your request without much time to stop it.