How to Add Pagination to Page?

I follow the instructions in https://www.wpbeginner.com/wp-tutorials/how-to-split-wordpress-posts-into-multiple-pages/ to add <!––nextpage––> tag to my page in classic editor, see below:

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However, the page is still not paginated, see http://www.sybase-recovery.com/test-pagination-2/

I check the HTML source of the page and see the following:

<p>test1<br /> &lt;!––nextpage––&gt;<br /> test2</p> 

Why?