Add in the url attachment loop the wp_link_pages of the post

by

In order to get the parent post, you would use the wp_get_post_parent_id() function.

So in your foreach loop, you would want to add something like:

$parent = wp_get_post_parent_id( $attachment->ID );
<a href="https://wordpress.stackexchange.com/questions/260459/<?php esc_url( the_permalink( $parent ) ) ?>">
   // Rest of your HTML
</a>