Somebody scammed Comcast by reporting the router as lost and is turning the router around to make money on it. Unfortunately you're the one they made the money on.
Comcast isn't going to sanction the seller's likely illegal actions by activating the modem now that they've already provided a replacement to the original owner.
Unfortunately your only option is to learn from this mistake that if a deal is too good to be true it's probably not true. You can buy a used one from a reputable seller for around $60 or a new one for about $85.
