This makes it possible to use `/page/` URLs with non-archive pages such as single pages in WordPress

wp_add_pagination_to_pages.php

<?php
// This makes it possible to use `/page/` URLs with non-archive pages such as single pages
add_action('init', 'add_pagination_to_pages');
function add_pagination_to_pages() {
  $pages = array(
    'flex/assessments'  => 220460,
    'flex/collections'  => 215682,
    'flex/lesson-plans' => 215685,
    'flex/my-standards' => 220768,
    'flex/resources'    => 220383,
    'flex/videos'       => 190851,
    'magazine'          => 150395
  );

  foreach ($pages as $path => $id) {
    add_rewrite_rule($path . '/page/?([0-9]{1,})/?$', 'index.php?page_id=' . $id . '&paged=$matches[1]', 'top');
  }
}
?>