$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 );
Don't forget to update the test meta key and test meta value by your meta key and meta value.