The “New Posts” sections is not a “page” per se, but a category section. Categories display the posts set to that category (and subcategories), not static content.
If you weren’t using the magazine layout, the workaround would have been to create a page using the “Category page with intro” template with the same slug “new-posts”, change the menu item to link to that page instead of the category, then add the banner to the “new-posts” page.
But the category page with intro template does not support magazine layout.