I found a reddit article detailing issues setting up port forwarding with this ISP. They had to call and get the ISP tech involved in opening ports. I would suggest giving the ISP a call, describing what you are doing and seeing how they can help.
Could you try another port number? I read on this forum that some ISP block some port numbers. In the Roon App under Settings → Roon Arc change the port number to ie. 44135 and change your port forwarding and see if that’s working.
It sounds like you’ve contacted your service provider and requested a routable IPv4 address. Your account diagnostics seem to confirm that your external IP is in this format. Provided there’s not an additional layer of carrier-grade NAT in play here, then I suggest you 1) restart your router/modem and all networking equipment, as well as your Core and then 2) try changing the port number in Roon → Settings → ARC as @Maarten_Duits helpfully suggested.
If that isn’t successful, please try disabling UPnP (if you’re setting a manual rule, you won’t need autoconfiguration via UPnP).
Thank you for the suggestion. Actually I was able to solve the problem by changing the setting of the Qnap internal firewall. Practically the router settings were blocked by the Qnap firewall, I just replicated the port forwarding setting in the Qnap firewall. Problem solved.