Verizon uses routers not modems. For Fios fiber optic connections.
call 1-800-VERIZON and they will swap it out for free if rented.
if purchased within 2 years they will also replace a purchased router.
please note that you want to make sure Verizon credits you with the exchange. They have been known to deny the return and try to charge you for it. But there should be no charge for the exchange.
if you have DSL it does use a modem a westell etc. which also should be free.