I have an Arris router (comcast) and I need to forward a port for a slingbox setup. Slingbox does not have Arris as a router manufacturer. If their is another manufacturer's router that is the same set up entering that router may also work. How do I obtain instructions to forward a port on an Arris TM804G/CT
The TM804 is a straight cable modem/eMTA and does not have a router component, so it does not do port forwarding. If you have an separate router attached to the TM804, that is the device you'd need to configure if port forwarding is required.