Sometimes, after connecting to a WiFi network with a captive login Mac OS doesn't show the login page. Depending on the captive configuration, it can be mitigated by opening a non-https page. The request will be captured and you will be redirected to the login page. However, it doesn't always work. In this case you can force display the MacOS captive login screen with this command:
open /System/Library/CoreServices/Captive\ Network\ Assistant.app