[Support request] Adding next previous buttons for (sub)pages not working

I have this code that used to work in the older version of the website, but I’m having difficulty to get it to work in GP.

It used to be inside page.php in the old theme, but I now added it using a hook element, execute PHP checked and the hook location I varied just to be sure I wasn’t making a stupid mistake (after header, before content, before main content etc.), and the display rule set to alle pages or entire site, nothing works. Adding a simple echo ‘hello’ works, so that is okay. The ID’s of the pages inside the code have not changed.

It should be visible on a subpage like this one: https://www.ullstorp.se/activities-available-here-or-near-ullstorp/swim-and-sunbathe-here-at-ullstorp/?lang=en

<?php
$pagelist = get_pages("child_of=".$post->post_parent."&parent=".$post->post_parent."&sort_column=menu_order&sort_order=asc");
$pages = array();
foreach ($pagelist as $page) {
   $pages[] += $page->ID;
}

$current = array_search($post->ID, $pages);
$prevID = $pages[$current-1];
$nextID = $pages[$current+1];
?>

<?php
if ( 5133 == $post->post_parent || 5135 == $post->post_parent || 6250 == $post->post_parent || 6252 == $post->post_parent || 6251 == $post->post_parent) {
?>

<div class="navigation">
<?php if (!empty($prevID)) { ?>

<div class="previous">
<a href="<?php echo get_permalink($prevID); ?>" title="<?php echo get_the_title($prevID); ?>"><?php echo get_the_title($prevID); ?></a>
</div>

<?php } if (!empty($nextID)) { ?>

<div class="next">
<a href="<?php echo get_permalink($nextID); ?>" title="<?php echo get_the_title($nextID); ?>"><?php echo get_the_title($nextID); ?></a>
</div>

<?php } ?>
</div>

<?php } ?>