Self-referencing canonical | WordPress.org

Hey, I have a problem with adding self-referencing canonical on a blog subpage. I tried different options. Plugins like Yoast, Rank Math, do not work, i.e. the option with canonical is active but canonical links are not added to the page code. I have tested various options and the only one that fairly works is to add the following code in the header.php:

<?php
	   if ( is_front_page() ) {
		  $canonical_url = get_home_url();
		  if ( ! is_main_site() ) { // add trailing slash for subsite home directories
			 $canonical_url = $canonical_url . '/';
		  }
	   } else {
		  $canonical_url = get_permalink();
	   }
	?>
	<link rel="canonical" href="https://projectdmc.org/support/topic/self-referencing-canonical-2/<?php echo $canonical_url ?>" />

The problem with this solution is that the blog subpage or categories from the blog as canonical have… the only article that has been published on the blog.

I have run out of ideas and I don’t know what to do about it. I would prefer to avoid installing any more plugins.

I would appreciate any help or guidance on what I can do to make it work.

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