Wordpress yoast seo plugin, generate custom sitemap for custom URLS

yoast-seo-sitemap.php

<?php
/**
 *  Create a new custom yoast seo sitemap
 */
 
add_filter( 'wpseo_sitemap_index', 'ex_add_sitemap_custom_items' );
add_action( 'init', 'init_wpseo_do_sitemap_actions' );


// Add custom index
function ex_add_sitemap_custom_items(){
	global $wpseo_sitemaps;
	$date = $wpseo_sitemaps->get_last_modified('CUSTOM_POST_TYPE');

	$smp ='';

    	$smp .= '<sitemap>' . "\n";
	$smp .= '<loc>' . site_url() .'/CUSTOM_KEY-sitemap.xml</loc>' . "\n";
	$smp .= '<lastmod>' . htmlspecialchars( $date ) . '</lastmod>' . "\n";
	$smp .= '</sitemap>' . "\n";


	return $smp;
}


function init_wpseo_do_sitemap_actions(){
	add_action( "wpseo_do_sitemap_CUSTOM_KEY", 'ex_generate_origin_combo_sitemap');
}




function ex_generate_origin_combo_sitemap(){


	global $wpdb;
	global $wp_query;
	global $wpseo_sitemaps;

        $post_type = 'CUSTOM_POST_TYPE';

	wp_reset_query();
	$args = array(
		'posts_per_page'   => -1,
		'orderby'          => 'post_date',
		'order'            => 'DESC',
		'post_type'        => $post_type,
		'post_status'      => 'publish',
		'suppress_filters' => true
	);
	query_posts( $args );
	wp_reset_postdata();

	//echo '<pre>';print_r($url);echo '</pre>';
	$posts_array = get_posts( $args );

	$output = '';
	if( !empty( $posts_array ) ){

		$chf = 'weekly';
		$pri = 1.0;

		foreach ( $posts_array as $p ) {
			$p->post_type   = $post_type;
			$p->post_status = 'publish';
			$p->filter      = 'sample';



			$url = array();

			if ( isset( $p->post_modified_gmt ) && $p->post_modified_gmt != '0000-00-00 00:00:00' && $p->post_modified_gmt > $p->post_date_gmt ) {
				$url['mod'] = $p->post_modified_gmt;
			} else {
				if ( '0000-00-00 00:00:00' != $p->post_date_gmt ) {
					$url['mod'] = $p->post_date_gmt;
				} else {
					$url['mod'] = $p->post_date;
				}
			}

			$url['loc'] = site_url().'/sample/all/'.$p->post_name;
			$url['chf'] = $chf;
			$url['pri'] = $pri;


			$output .= $wpseo_sitemaps->sitemap_url( $url );

			// Clear the post_meta and the term cache for the post, as we no longer need it now.
			// wp_cache_delete( $p->ID, 'post_meta' );
			// clean_object_term_cache( $p->ID, $post_type );
		}
	}

	// Grab last modified date
	$sql  = $wpdb->prepare(" SELECT MAX(p.post_modified_gmt) AS lastmod
		FROM	$wpdb->posts AS p
		WHERE post_status IN ('publish') AND post_type = %s ", $post_type );

	$mod = $wpdb->get_var( $sql );

	// Generate terms URLs
	$practitioner_terms = get_terms( 'TAXONOMY', 'orderby=count&hide_empty=0' );
	if( !empty( $practitioner_terms ) ){

		$pri = 1;
		$chf = 'weekly';

		foreach ($practitioner_terms as $key => $term ){

			$url = array();

			$url['loc'] = site_url().'/sample/'.$term->slug;
		      	$url['pri'] = $pri;
		      	$url['mod'] = $mod;
		      	$url['chf'] = $chf;
		      	$output .= $wpseo_sitemaps->sitemap_url( $url );

		}
	}

	// Generate permutation & combinations
	if( ( !empty( $practitioner_terms) ) &&  ( ! empty($posts_array ) ) ){

		$pri = 1;
		$chf = 'weekly';

		wp_reset_postdata();


		foreach ($practitioner_terms as $key => $term ){

			$args = array(
				'posts_per_page'   => -1,
				'orderby'          => 'post_date',
				'order'            => 'DESC',
				'post_type'        => $post_type,
				'post_status'      => 'publish',
				'suppress_filters' => true,
				'tax_query' => array(
				    array(
				        'taxonomy' => 'TAXONOMY',
				        'field'    => 'slug',
				        'terms'    => $term->slug,
				        'operator' => 'IN'
				    ),
				),
			);

			$posts_array = get_posts( $args );
			//echo '<pre>';print_r($posts_array);echo '</pre>';

			$url = array();

			foreach ($posts_array as $key => $p ){
				$url['loc'] = site_url().'/sample/'.$term->slug.'/'.$p->post_name;
			      	$url['pri'] = $pri;
			      	$url['mod'] = $mod;
			      	$url['chf'] = $chf;
			      	$output .= $wpseo_sitemaps->sitemap_url( $url );

			}

		}
	}


        if ( empty( $output ) ) {
            $wpseo_sitemaps->bad_sitemap = true;
            return;
        }

        //Build the full sitemap
        $sitemap = '<urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" ';
        $sitemap .= 'xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" ';
        $sitemap .= 'xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">' . "\n";
        $sitemap .= $output . '</urlset>';

        //echo $sitemap;
        $wpseo_sitemaps->set_sitemap($sitemap);

}

/*********************************************************
 *  OR we can use $wpseo_sitemaps->register_sitemap( 'CUSTOM_KEY', 'METHOD' );
 ********************************************************/

add_action( 'init', 'ex_register_my_new_sitemap', 99 );
/**
 * On init, run the function that will register our new sitemap as well
 * as the function that will be used to generate the XML. This creates an
 * action that we can hook into built around the new
 * sitemap name - 'wp_seo_do_sitemap_my_new_sitemap'
 */
function ex_register_my_new_sitemap() {
	global $wpseo_sitemaps;
	$wpseo_sitemaps->register_sitemap( 'CUSTOM_KEY', 'ex_generate_origin_combo_sitemap' );
}

add_action( 'init', 'init_do_sitemap_actions' );

function init_do_sitemap_actions(){
	add_action( 'wp_seo_do_sitemap_our-CUSTOM_KEY', 'ex_generate_origin_combo_sitemap' );
}

I am getting 404 - page not found in my staging environment after generating custom sitemap using this code. Please suggest a fix for this. I have already saved permalink, cleared all kind of cache but still not working.