How to show related posts of category on post within custom posttype

@ricardio-de-penning
I think you are going on wrong direction.
You just want to get only those posts which is related to current category right?

If yes then you need to filter post using tax_query like below.

$args = array(
    'post_type' => 'post',
    'tax_query' => array(
        array(
            'taxonomy' => 'category',
            'field'    => 'term_id',
            'terms'    => array($cat_ids),
            'operator ' => 'IN',
        ),
    ),
);
$query = new WP_Query( $args );