In WordPress we can separate post/pages using <!--nextpage-->
. It’s hard to remember the tag, and you need to add it using “Text” editor, not visual editor. This is how you can add “Next Page” or “Page Break” button in Visual Editor, Just like this:
It’s very simple, you can add this code in your theme/child theme functions.php or add it in a functionality plugin. Read More How to add “Next Page”/ “Page Break” button in WordPress Editor