centos7 linux laptop can't connect to public wifi portals

It's not always the case, but sometimes/often I can't connect to public wifi portals from my linux laptop that open a login page automatically. The page is never displayed, and I get no internet service (though there is a wifi connection and network setup). This always happens on American Airlines and Delta flights, which is nice.

I did some research and found a couple of threads suggesting that I need to configure the NetworkManager service, and it will open the login page on the captive wifi portal. I found another link on the matter, with details about how to edit the config file. I basically did what it says, though I called the file /etc/NetworkManager/conf.d/20-connectivity-centos.conf. Supposedly this is read by the NetworkManager daemon at startup.

I restarted the daemon using these instructions, since the ones in the ubuntu threads don't work for me. I used "sudo systemctl restart NetworkManager".