Open Ajax Dialog Boxes routes in administration theme

Ok, so i found the solution: you have to force the admin theme in the drupalSettings ajaxPageState variable.
Like this:

/**
 * Implements hook_js_settings_alter().
 */
function MODULENAME_js_settings_alter(array &$settings, \Drupal\Core\Asset\AttachedAssetsInterface $assets) {
  $settings['ajaxPageState']['theme'] = \Drupal::config('system.theme')->get('admin');
}

@jmauricio if you've answered your own question can this then be closed