If you are using menu order for CTP post index and viewing single then you can use this snippets to get the right post link for next/prev.

get_previous_next_post.php

<?php 
function mondira_previous_post_where( $in_same_term, $excluded_terms ) {
	global $post, $wpdb;
	if ( get_post_type( $post ) == 'portfolio' ) {
		return $wpdb->prepare( "WHERE p.menu_order < %s AND p.post_type = %s AND p.post_status = 'publish' $posts_in_ex_terms_sql", $post->menu_order, $post->post_type);
	}
}
add_filter( 'get_previous_post_where', 'mondira_previous_post_where', 10, 2 );
 
function mondira_next_post_where( $in_same_term, $excluded_terms ) {
	global $post, $wpdb;
	if ( get_post_type( $post ) == 'portfolio' ) {
		return $wpdb->prepare( "WHERE p.menu_order > %s AND p.post_type = %s AND p.post_status = 'publish' $posts_in_ex_terms_sql", $post->menu_order, $post->post_type);
	}
}
add_filter( 'get_next_post_where', 'mondira_next_post_where', 10, 2 );
 
function mondira_previous_post_sort($order_by) {
	if ( get_post_type( $post ) == 'portfolio' ) {
		return "ORDER BY p.menu_order desc LIMIT 1";
	} else {
		return $order_by;
	}
}
add_filter( 'get_previous_post_sort', 'mondira_previous_post_sort' );
 
function mondira_next_post_sort($order_by) {
	if ( get_post_type( $post ) == 'portfolio' ) {
		return "ORDER BY p.menu_order asc LIMIT 1";
	} else {
		return $order_by;
	}
}
add_filter( 'get_next_post_sort', 'mondira_next_post_sort' );
?>