You’re using category query for your custom taxonomy. So it won’t work, because there are no categories with such IDs.
You should use tax_query instead.
// post types query
while ( have_posts() ) {
the_post();
// getting the categories
$categoryIDs = array(); // it's much nicer than concatenated string
$getcategory = get_the_terms($post->ID, 'custompostnamehere-categories');
foreach($getcategory as $t) {
$categoryIDs[] = $t->term_id;
}
// end of post type query
}
// resetting query isn't needed - you will call your own query next
//starting new query to get related posts within the same categories
$args = array(
'post_type' => 'custompostnamehere',
'tax_query' => array(
array(
'taxonomy' => 'yourcustomtaxonomy',
'field' => 'id',
'terms' => $categoryIDs
)
),
'posts_per_page' => 20
);
$q = new WP_Query($args);
// retrieving the data
while( $q->have_posts() ) {
$q->the_post();
...
}