Hathway — Hathway sending me notices to pay for modem which is returned after 8 years.

I used hathway for about 5 months and disconnected in November 2015 in Pune. I did all the right things called the office before I moved out of the house. The technicians assured me there is no need to return the router and will give connection to next tenant. They told me not to worry about the router. Thereafter I was sent invoices after 2 years to pay some made up bills. After complaining to their help desk and told them I will take legal action they stopped send me invoices. Now after 9 years they are sending me bill to pay for router. It is ridiculous to deal with incompetent people.
My account number is 1378369.