@terziyski 

It doesnt look like uPNP is enabled, it seems the X20 doesn't have a firewall unless you pay for the Homeshield Pro subscription service.

  @TeeDee1980 

From Do I Need a Firewall if I Have a Router?: "Home routers use network address translation (NAT) to share a single IP address from your Internet service provide among the multiple computers in your household. When incoming traffic from the Internet reaches your router, your router doesn’t know which computer to forward it to, so it discards the traffic. In effect, the NAT acts as a firewall that prevents incoming requests from reaching your computer."

Without setting port forwarding, enabling UPnP, or configuring DMZ - how would X20 router know to forward ssh traffic to your NAS specifically?

  @Alexandre. 
 

You raise a very good point, and I do use port forwarding on the NAS for other functions such as an occasional VPN client, but ssh isn't one of them and the NAS is listening on the default port. Sadly the options for configuring or looking at things like log files are minimal. I since took out a 30 day trial for the HomeShieldPro subscription and whilst that may plug the hole, it still doesn't provide any real management facilities. It's all a bit Mickey Mouse really, I plan on buying a proper firewall, using the X20 just in AP mode and canceling the ridiculous subscription.