Ok, so you want to get all posts type competitions that share the same brand as the current post? If so, you would need a meta_query:
https://www.advancedcustomfields.com/resources/query-posts-custom-fields/
$query = new WP_Query(array(
'post_type' => 'competitions',
'posts_per_page' => 5,
'post_status' => 'publish',
'meta_key' => 'related_brand',
'meta_value' => get_field('related_brand') // This is related brand of current post.
));
See if the query returns anything back?
