[Kymmy]

If NAT is disabled then the router is simply being used as a modem/hub.. for internal IP's you should then have a 2nd cable style router (ethernet router) on one of the external IP's to provide you with a secure internal IP location.

Without the 2nd router you'll instead need to put the NAS directly on an external IP but as it's still behind the firewall it should be safe if you only open up the ports you need and do a default DENY to the IP for all other ports