Get Grandparent’s Title

Hi,

Please kindly help.

It seems that I can’t get the grandparent’s title of CPT posts.

I already use the below code, but so far I only get the parent post title.

<?php

$current = $post->ID;
$parent = $post->post_parent;
$grandparent_get = get_post($parent);
$grandparent = $grandparent_get->post_parent;
echo get_the_title($grandparent);

?>

This is my CPT’s settings.

name: series
label: Series
singular_label: Serie
description: ""
public: true
publicly_queryable: true
show_ui: true
show_in_nav_menus: true
delete_with_user: false
show_in_rest: true
rest_base: series
rest_controller_class: ""
rest_namespace: ""
has_archive: true
has_archive_string: ""
exclude_from_search: false
capability_type: post
hierarchical: true
can_export: false
rewrite: true
rewrite_slug: ""
rewrite_withfront: true
query_var: true
query_var_slug: ""
menu_position: ""
show_in_menu: true
show_in_menu_string: ""
menu_icon: dashicons-welcome-view-site
custom_supports: ""
enter_title_here: Add Serie