When I site is set to ‘login required’, there is no way to allow non-registered users to see any of its content through the Discourse user interface. To grant access to some of your site’s categories to non-registered users while at the same time preventing non-registered users from seeing content in other categories, you will need to first setup category permissions that limit access to the categories that you want to protect. Then you will need to disable the ‘login required’ site setting to open the site up to non-registered users.
For an overview of category security settings, see [Video] How to create a private category using group permissions.