$post_args = array(
  \'post_type\'              => \'post\',
  \'posts_per_page\'         => -1,
  \'meta_key\'               => \'field_name\',
  \'meta_value\'             => \'field_value\',
);

$post_query = new WP_Query( $post_args );
$posts_array= array();
if ( $post_query->have_posts() ) {
    while ( $post_query->have_posts() ) {
        $post_query->the_post();
        $posts_array[] = get_the_ID(); //Array of post ids
    }
    wp_reset_postdata();
}

//YOUR COMMENT ARGS SHOULD BE THIS
$args = array(
    \'number\'         => \'30\',
    \'order\'          => \'DESC\',
    \'orderby\'        => \'comment_date\',
    \'post__in\'        => $posts_array, //THIS IS THE ARRAY OF POST IDS WITH META QUERY
);

$comments_query = new WP_Comment_Query;
$comments = $comments_query->query( $args );