First try to reach the server locally from a different device to see if a firewall blocks that port.If you can reach the router with e.g. a smartphone without wifi connected, you know that duckdns and forwarding also works for you. This is not the case for all internet providers. Especially with IPv6 -> IPv4 routing it may be impossible.Then it should only depend on the right configuration of the router.
When I said locally I mean with the local IP (192.x.x.x or 10.x.x.x) of your computer from inside your intranet. Often windows blocks the port until you say to allow external access.Then accessing external ip from intranet is the next test. Since you reach that directly from inranet I expect this to work with right forwarding enabled.