So, the Marantz has an ethernet cable that's connected to the Arris router, right? Was the Arris new? Your computer is connected to it and working, right? Did you do any setup for it?
If you move the Marantz to the Arris router and use the same ethernet cable being used by the computer, it should work. If not, the Marantz has a problem internally, or in the setup.
Your computer should work using any of the four ports on the Arris- if any don't work, something is set up incorrectly and it may be necessary to reset the router to its default, especially if you didn't get a new one and you didn't create the WiFi name/settings. You can also connect the existing Marantz ethernet cable to Port 1 of the Arris to see if that makes a difference. If this works and none of the other three work, the Arris needs to be reconfigured.