You’ll need to setup the port forwarding to port all requests to port 5000 to be forwarded to the internal IP.
It’s not recommended to use a Windows server to do your routing. I would recommend getting a router as it’s got NAT, firewall, and other security features built in. Most routers also include port forwarding to handle this.
While ISA can handle this, you’ll want something infront of it to handle firewalling the Windows server.
By having your server on the Internet directly you are open to attack by hackers, virus’, etc.