RT-N66U L2TP forwarding problem

[vaio6]

I have a Synology NAS with VPN, I forward the following port to Synology but it fails to connect. Ports forwards are

1701
500
4500

I switched the router to airport extreme, it works again. It is therefore not the setting of Synology, did I miss something with Asus setting?

Anyone can help?

Thanks