Let’s try the “Read more” insert. I’ll create the post in text (HTML) and insert “Read more” within this editing environment, which should avoid formatting errors that can occur if it’s inserted within the visual mode
(it can get inserted within HTML formatting). So let’s write a lot more and see if we can get the right break and the right presentation. Answer: it still doesn’t work. Is it a problem that the theme doesn’t support it? Look for a page where it works and inspect the source code.