Pagination not working with custom post type

Question

From this answer, I now have an adjusted WP_Query statement (I added the $paged variable):

$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$loop = new WP_Query( array( 'post_type' => 'laptop', 'postsperpage' => '30', 'max_num_pages' => 20, 'paged' => $paged) );

if ( $loop->have_posts() ) { ?>
   ...output content....
} ?>

<?php if ( $loop->max_num_pages > 1 ) : ?>
<div id="nav-below" class="navigation">
  <div class="nav-previous">
    <?php next_posts_link( __( '<span class="meta-nav">&larr;</span> Previous', 'domain' ) ); ?>
  </div>
  <div class="nav-next">
    <?php previous_posts_link( __( 'Next <span class="meta-nav">&rarr;</span>', 'domain' ) ); ?>
  </div>
</div>
<?php endif; }
   wp_reset_postdata();
?>

Nothing is output in <div class="nav-previous"> or <div class="nav-next">.

Can you see what is wrong here please?

Answer 1

From WordPress Codex:

Add the $max_pages parameter to the next_posts_link() function when querying the loop with WP_Query. To get the total amount of pages you can use the 'max_num_pages' property of the custom WP_Query object.

So:

next_posts_link( __( 'Next <span class="meta-nav">&rarr;</span>', 'domain' ), $loop->max_num_pages );