Why Android devices don't display Captive Network Assistant (CNA)?
When the strict policy is set to 'Allow HTTPS traffic
without sign-on', the captive portal does not show up automatically because from
Android OS 7.1 or newer, Android devices will use some HTTPS pages to test
connectivity when detecting captive portals in wireless networks. As
the HTTPS traffic is allowed, the captive portal landing page pop-up (CNA) will
not be displayed.
So, we suggest setting the captive portal’s strict policy to 'Block all access until sign-on' allowing the captive portal page to pop-up when clients connect to the SSID.