No no;
I'm just saying that the incoming firewall rules you added before with source interface WAN are not necessary IF NO port forwarding has been configured.
I gess your nas is not in DMZ, so if no port forwarding is configured with the NAS as des...
Well if you don't have port redirection rules from internet to your nas, the incomming firewall rule is useless, no traffic is redirected from the WAN port to you NAS.