Recently, after upgrading the Qnap NAS to QTS 5.0.X , there was a problem of not being able to access to the web page from internet. However, accessing from LAN still works. So at the beginning, I thought it might be a firewall or port forwarding issue.
My network topology is a router with public IPs attached, allowing incoming 5000 and 5001 ports from WAN, and forwarding them to the NAS’s IP. It has been in use for over a year, but recently after the system upgrade, this problem occurred, when accessed the web page, it prompted connection reset.
The solution is simple, QTS 5.0.X disabled the router UPnP service by default (for security reasons). Follow this official doc to re-enable UPnP service and restart NAS should fix this.