So this is just converting your cable line in to an RJ45 I believe.
You're putting the cart before the horse; Internet isn't the problem currently if you cant take control of the unifi hardware. Plug your USG directly in to the Arris, plug the switch in to the LAN on the USG, and set up everything else as per normal.
Then, use the Unifi software/controller on your PC. It should see all unifi hardware as unadopted. Adopt it, and configure it all through the unifi software.
See this for more info.
You'll have to set up the USG through this software as per your ISP's cable configuration setup. I dont have cable so I'm not sure exactly how this is done for your ISP; See
this for specific USG help setup.