prev-next-posts-loop.php

<?php
function rl_cold_next_post_id( $post_id, $post_type ) {
	global $post;
	$oldGlobal = $post;
	$post = get_post( $post_id );
	$previous_post = get_previous_post();
	$post = $oldGlobal;
	if($previous_post == ""){
		return rl_cold_new_post_id($post_type);
	}
	return $previous_post->ID;
}
function rl_cold_new_post_id($post_type) {
	$rl_cold_args = array(
		'post_type' => $post_type,
		'post_status' => 'publish',
		'posts_per_page' => 1,
		'order' => 'DESC',
	);
	$rl_cold_team_post = new WP_Query($rl_cold_args);
	while($rl_cold_team_post->have_posts()){
		$rl_cold_team_post->the_post();
		return get_the_ID();
	}
	wp_reset_query();
}


function rl_cold_previous_post_id( $post_id, $post_type) {
	global $post;
	$oldGlobal = $post;
	$post = get_post( $post_id );
	$next_post = get_next_post();
	$post = $oldGlobal;
	if($next_post == ""){
		return rl_cold_old_post_id($post_type);
	}
	return $next_post->ID;
}
function rl_cold_old_post_id($post_type) {
	$rl_cold_args = array(
		'post_type' => $post_type,
		'post_status' => 'publish',
		'posts_per_page' => 1,
		'order' => 'ASC',
	);
	$rl_cold_post_post = new WP_Query($rl_cold_args);
	while($rl_cold_post_post->have_posts()){
		$rl_cold_post_post->the_post();
		return get_the_ID();
	}
	wp_reset_query();
}