Most likely, your Custom Post Types are using custom taxonomies that happen to be named “Category”. Try something like this:
$args = array(
'post_type' => $cpt,
'tax_query' => array(
array(
'taxonomy' => '[whatever your taxonomy is named],
'field' => 'slug',
'terms' => 'upcoming-meetings',
)
)
);
$query = new WP_Query( $args );
You may be able to find out the taxonomy name from a management screen in the backend. I’m not familiar with the Custom Post Types UI plugin, so unfortunately I can’t really help you there.
Definition
“Taxonomy” is a catch-all term that encompasses WordPress’s native Categories, Tags, and any custom taxonomies that you (or a plugin) may create. So “Category” is a type of taxonomy, as is “Tags”.