Able to join captive WiFi network, but not connect to internet
One more thing that you may try that works fairly consistently for me. Under Network Preferences, change Location to some other, if this is the first time you do this, do "Edit Locations" and press the "+" sign and add one using any name you like, copy any old settings that may apply. Apply them, and "Hey, presto!" you get the log-in screen of your internet service.

Hope this helps.