Add Custom Related Products Globally to all the Related Products

functions.php

<?php 


// Add the following code on your theme or child theme functions.php file - 


if( !function_exists('wpb_wrps_add_custom_related_products') ){
	function wpb_wrps_add_custom_related_products( $args ) {

		global $product;
		$number_of_related_products 	= wpb_wrps_get_option( 'wpb_wrps_number_of_products', 'wpb_wrps_general', 100 );
		$default_related 				= apply_filters( 'wpb_wrps_related_products_ids', wc_get_related_products( $product->get_id(), $number_of_related_products, $product->get_upsell_ids() ));
		
		$args['post__in'] 				= array_merge( $default_related, array(14, 20, 25) ); // Your custom related products ids here

		return $args;

	}
}
add_filter( 'wpb_wrps_related_products_args', 'wpb_wrps_add_custom_related_products' );