How to spawn "sign-in to network" page for a captive portal? How do I execute captive portal?

On a Z828 from ZTE, Android version 5.1.1, I'm trying to get something like:



While Starbucks generally works for me, all too often, other networks, when using public Wi-Fi, simply give a notice along the lines of:



See also:

Can I get a notification of failed WiFi connection for secured network?

Notify when connected to working Wi-Fi point

Only connect to certain Wi-Fi networks

Nearby wifi networks causing notification

How to disable the &quot;sign in to wifi network&quot; page?

Although nothing is quite on point, leaving me at:


  Still can't connect?
  
  If you still can't connect, please contact your Internet Service
  Provider, router manufacturer, or network administrator.


Which, simply isn't an option for most public Wi-Fi hotspots.  Perhaps, there's simply no solution.

Incidentally, my Kindle connects to the Wi-Fi hotspot, then, of its own volition, brings me to the captive portal through datavalet.  Seems specific to Android that I must first try to browse to a random website, which, sometimes, will then bring me to a captive portal.  Maybe.
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.