For DOCSIS, you really have to use the modem provided by Hathway as it has to be registered in the backend. You cannot use your own modem. Call them and tell them you threw it away you may have to pay them some fee but they should provide a new one.
Atleast on DEN, they consider the modem as their property. 3 months after I stopped recharging DEN, they came to my home to pick up the modem.