Query Posts with Custom Taxonomy from a Custom Post Type

It looks like you forgot to set the field parameter to search for slugs:

$args = array(
    'post_type' => 'advertisement',
    'tax_query' => array(
        array(
            'taxonomy' => 'group', 
            'terms' => array( 'homepage' ),
            'field' => 'slug',
        )
    )
);

Notice that the taxonomy query uses the term_id as the default search field.

I would also use another variable than $wp_query to store the result of the secondary query.

I hope this solves your problem 😉