Synology VPN client on iPhone allows ipv6. And, you can't turn off ipv6 on the iPhone's cellular connection. Which means: If you're running Synology VPN server on your router, and your router's connection for whatever reason doesn't have an ipv6 address enabled, then the iPhone will just bypass the VPN except when communicating with LAN resources on your router's network. So, your iPhone's internet traffic is unprotected by the VPN. Kind of annoying. It would be nice if the Synology VPN client app allowed you to disable ipv6 in the app (as the OpenVPN app does), or at least warned you that your connection was unprotected.