Did the tool show the port was open? I assume you are running IIS somewhere and I can remember were and dont have it in front of me you set the ip for IIS to bind to if it bound to the other nic it wont answer if the port checker is showing the port open and you dont have any firewall blocking it setting the binding should fix it.
You could try this mini free webserver very simple to set up try that just to test if the port is working if it work with this you know its your webserver problem its a great little server very small fast and free and very simple to use.

Are you getting someone external to test this for you as a 99% of routers wont work on anything connected internally.

It is high risk running a webserver on a DC. I would try the mini webserver on a pc and forward to that unless you need a DC