Override page template and preserve administration navigation bar
What the code is missing is the code that sets those variables the template file is using. Since you are using page--mymodule as template name, the module needs to implement template_preprocess_page__mymodule().
Alternatively, you can suggest a template file using hook_theme_suggestions_page().
function my_module_theme_suggestions_page(array $variables) {
  $path = explode('/', trim(\Drupal::service('path.current')-&gt;getPath(), '/'));
  if (isset($path[0]) &amp;&amp; $path[0] == 'hello') {
    return 'page--mymodule';
  }
} 

Implementing hook_theme_suggestions_page() like I show, page--mymodule.html.twig will get the variables set in template_preprocess_page(), which are the same variables passed to page.html.twig.