The key thing is your router
You need to open a forwarded port from that to the NAS device, and preferably put the device into the "DMZ" with a fixed IP address
That way you can access the NAS device from inside and outside, with less risk of anyone who acccesses the router from outside being able to break into the network
Of course yo need to password the router as well.....
So first question:
What router do yo have> And can it handle incoming port forwarding and the allocation of an IP address to a "DMZ" (so-called demilitairzed zone)