Connect NAS from the internet

What is the best way to provide access from the internet to a NAS that is behind a router that is connected to a Zyxel VPN 100? 

This is the use case: Several users of smartphones and computers on the internet would like to access data on a NAS. The NAS is connected to a router, I assume it's a Fortigate device. This router has a fix IP address on the WAN port and connects to a Zyxel VPN 100, that has two WAN links.

Thanks for any hints

Peter