My installer was by the book. Him hooking the cable modem to the router would have triggered a monthly home networking charge.
I wound up having him leave with just the "one computer" attached directly to the modem. I then reconnecting everything the way I desired. (Modem>router>various-devices)
I did have to do two things, remove the Comcast software that had been installed on the "one computer", and configure the router to spoof the MAC address of the "one computer".
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Glenn