Why the page number does not appear in splitting post?

I just follow the instructions in https://www.wpbeginner.com/wp-tutorials/how-to-split-wordpress-posts-into-multiple-pages/#without-plugin to split a large post into several pages. However, after inserting the page breaks, when loading the post, I can only see the first page, and there are no page numbers like Pages: 1 2 3 4 appear in the footer, why? Do I do something wrong?

My test page is at https://www.datanumen.com/file-splitter/errors/test-error-pagination/

Thank you.