It is the android way of detecting if a Captive Portal sign in is required .
The detection url differs between different Android versions.For Marshmallow, it uses connectivitycheck.gstatic.com/generate_204, check the code hereAndroidXRef - Marshmallow 6.0.0_r1
For Kitkat, it uses clients3.google.com/generate_204 check the code .
To avoid these popping of webpages we can add these below urls inro whitelist on access point .
I'm having the same issue. I thought it was because I tried the Q beta and then downgraded back to P... but I've factory reset several times now and the issue persists. 9AppsVidMateapp.vinVLC
Can you check what does the DNS settings shows up on your phone. Based on the error it looks like the DNS is failing for you. I hope it has not enabled secured/Private DNS on this phone.
Android and iPhones should open the Captive Portal Page automatically. Only when whitelist specific websites, we don't get Portal page automatically. These specific websites get whitelisted by default when we enable Social login features (Facebook login,Google login) . In that case we get Captive Portal page only when we browse any http website .Please send the tech-support file to Sandeshkumar.b@cambiumnetworks.com so that we can debug the issue .