Yes, this is normal. On your local network, the NAS keeps the ports 80 and 443 to work. When a connection comes from the outside though, on the ports 9080 or 9443 of your router WAN side, your router will forward the traffic to your NAS on the corresponding ports.
Make sure you have set up the port forwarding in the router, and that you have specified in the NAS which ports your have setup.
