You cannot just uncheck and save. You need to uninstall it from "Uninstall" tab in "Extend" page
- Install Disqus module
- Module configuration add the shortname you created in disqus.
- Give "View Disqus comments" permission to anonymous user and remove core comment permissions.
- Now this is the key part. In Drupal 8 comments comes as fields. So you need to add it as a field again. BUT IT DOES NOT APPEAR UNLESS YOU REMOVE CORE COMMENT FIELD IN MANAGE FIELDS PAGE.
- So remove old comments field and add Disqus comment field