nextpage to split content and paginate pages /posts doesn't work

Hello.

While there are many threads helping people to use wordpress category pagination, I couldn’t find one about troubleshooting the tag <! – – nextpage – – > needed to split and paginate pages or posts with lots of content.

My problem is the “nextpage tag” simply doesn’t work. It truncates the article at the point I add it, but doesn’t show any navigation.

I have a custom theme applied to a live site wich I can modify but not replace, so originally I thought it was a theme problem.

But now, I’m using a VERY simple page template to test it:

<?php
/*
Template Name: prueba
*/
?>
<?php get_header() ?>

<?php
if (have_posts()) :
	while (have_posts()) :
		the_post();
		the_content();
	endwhile;
endif;
?>

<?php get_footer(); ?>

Even with this simple structure, the problem persists, and I don’t know its reason. Could it be something wrong in the Functions.php?

I’m sure there’s something I’m overlooking.

Please help.