Maintenance Mode try

maintenance.php

<?php

public function onKernelRequest(GetResponseEvent $event)
{
    if (!$event->isMasterRequest()) {
        return;
    }

    if ($this->isMaintenanceMode() && !$this->isUserAdmin()) {
        $event->setResponse($this->renderMaintenanceView());
    }
}

private function isMaintenanceMode()
{
    $setting = $this->settings->findByCode('maintenance_mode');

    return null !== $setting && 1 === (int) $setting->getValue();
}

private function isUserAdmin()
{
    return $this->checker->isGranted(AdminUserInterface::DEFAULT_ADMIN_ROLE);
}