NAS Hard-drive Install/Configure with Orbi/AT&T

I have the #RBR750 Orbi with 3 RBS750 Satellites running behind an AT&T Modem BGW320-505 configured in passthrough. It is working great with the 1 Gbps Fiber to the home offering from AT&T

I am planning to purchase an 8TB NAS and attach to the network as MyCloud and get to access the drive from the LAN *and* the WAN (i.e. outside of my network). I understand that the USB drive for the  AT&T BGW320-505  modem will not work, hence need to buy a NAS.

Is the above doable? Can I access the NAS from outside the firewall and from the cloud? Can I attach the NAS to one of the Satellites instead (I ran out of Ethernet ports on my RBR750 base-station Orbi)?

Anyone has set-up the above and working OK? Any recommendation on best and most cost-effective 8TB NAS on the market now?