A 2800 series router is a commercial router. Because it is a real router and not a "gateway" it has a large number of options that a consumer router does not. It makes it much more complex to configure.
Since IOS on cisco has been around well before consumer router existed it is not called port forwarding or DMZ. These words assume you only have a single IP external IP that can be natted to a internal ip.
In IOS it is called a static nat entry, which exactly you need depends if you are attempting to share a IP with the router and the device or if you have a IP to dedicate to the server.
The command you want is a one of the variations of the command
IP NAT INSIDE SOURCE STATIC ...........