/**
* Define conditions when the URL should be publicly accessible.
*
* @return bool
*/
function should_be_accessible() {
return defined( DOING_AJAX ) || defined( DOING_CRON ) ||
    is_user_logged_in() || is_admin() ||
    $GLOBALS['pagenow'] === 'wp-login.php';
}

/**
* Turn on basic maintenance mode if the URL should not be publicly accessible.
*/
function maintenance_mode() {
if ( should_be_accessible() ) {
 return;
}

wp_die( 'Maintenance mode.' );
}
add_action( 'init', 'maintenance_mode' );