Show Custom Post Types in archive page

Change ‘your-custom-post-type’ to your desired custom post type.

You should use $query->is_main_query() to only show these in your archive page (if you don’t, your custom post type posts may show up in your sidebar widget, etc.).

function hoge_show_cpt_archives( $query ) {

    if( is_tag() &&  $query->is_main_query() ) {
        $query->set( 'post_type', array(
            'post', 'your-custom-post-type'
        ));
        return $query;
    }
}
add_filter( 'pre_get_posts', 'hoge_show_cpt_archives' );