I got this working. It's working method is not the way I anticipated. I had given all in the user groups section. When I selected a specific user group, portal started to display for end-users.
that is indeed more or less what the article says:
"When configuring a captive portal through the CLI, you may set security-groups to a specific user group. The result of this configuration will show an authentication form to users who wish to log in to the captive portal—not a disclaimer page. If you do not set any security-groups in your configuration, an "Allow all" status will be in effect, and the disclaimer page will be displayed for users."
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