you were asked for the router back, but did they demand it back or did you protest?
when I canceled cablevision they said they wanted their modem back which I had purchased.
What do you mean by "demand" vs "ask"? When I was closing account I asked what equipment they want me to return (if any), they said just return the router and keep battery and ONT. I never paid for the router (I don't recall if I had a charge and credit on my original bill like OP but I definitely didn't pay for it), I had no use for the router anyway, why would I protest? If I wanted it they are available on eBay for $30.