Welcome to Meta!
Since you enabled login required
, I can only see a login page. As long as your whole site is login required, this overrules your category security settings. You can customize the login page, for example, with Trendy Login. Combined with Page Publishing to publish your topic explicitly also for not logged-in users, you could show a link to your topic on your login page.
There is also a theme component to set Custom Homepage for Groups. But I don’t think it works for anonymous users when login is required.