How To Use Related Posts by Taxonomy Plugin To Display Related Content Below Posts

If you want to use related posts by taxonomy plugin to display list of related posts automatically after your WordPress posts, you will need to add a code snippet in your (child) theme’s functions.php file.

Here is an example with four related posts after the content on all single post pages.

Note: See below this code snippet how you can change it to suit your needs.

add_filter( 'the_content', 'add_related_posts_after_post_content' );
function add_related_posts_after_post_content( $content ) {
    //check if it's a single post page.
    if ( is_single() ) {
        // check if we're inside the main loop
        if ( in_the_loop() && is_main_query() ) {
            // add your own attributes here (between the brackets [ ... ])
            $shortcode = '[related_posts_by_tax posts_per_page="4"]';
            // add the shortcode after the content
            $content = $content . $shortcode;
    return $content;

Source : The plugin’s doc