You do not want anything that is classified as a "wifi repeater" -- you're looking simply for a routed configuration using a wireless uplink (i.e. wireless WAN, or WWAN).
You'll start with a base configuration of OpenWrt and configure your LAN as you desire. Then, connect to the upstream network using the general method linked here: (but there's a better way... described in a moment):
The better way, though, is to use Travelmate to manage your upstream connections. It works really well for this use case.
Are you actually traveling with your NAS? If not, are you expecting to be able to access it while on the go via a remote connection (such as a VPN)?
There are smaller, purpose built travel routers. You might want to consider those. But otherwise, sure, this unit is fine.