$args = array(
      'post_type' => 'reserveringen',
    );

    $reservation_query = new WP_Query( $args );
     if ( $reservation_query->have_posts() ) : ?>
      <ul>
        <!-- the loop -->
        <?php while ( $reservation_query->have_posts() ) : $reservation_query->the_post(); ?>
          <li><?php

          $user_ip_db = getUserIP();
          $ipadres = get_post_meta($post->ID, 'user_ip_reservering', true);
          $datum_nieuwe_reservering = get_post_meta($post->ID, 'datum_nieuwe_reservering', true);

          echo $ipadres . ' - ' . $datum_nieuwe_reservering . '<br>';

        ?></li>
        <?php endwhile; ?>
        <!-- end of the loop -->
      </ul>

      <?php wp_reset_postdata(); ?>

    <?php else : ?>
      <?php echo 'Sorry, er zijn geen reserveringen <br>'; ?>
    <?php endif;
    ?> <!--END OF SHOW ALL CUSTOMERS SELECTION -->