Template suggestion doesn't work with a custom menu link (custom route)
I'm guessing you put the template in your module folder and not in a theme?

You need to register the template with something like this in your .module file:

/**
 * Implements hook_theme().
 */
function mymodule_theme() {
  $theme['page__mycustomtemplate'] = [
    'template' =&gt; 'page--mycustomtemplate',
  ];

    return $theme;
}