I would sugest calling again and cancel your contract giving one months notice and at same time sign up for the service again as a "new" customer asking for the free instalation to take place in one months time.
If nothing else it will point out just how ludicrous this situation is. LOL
Other comical option might be to send them an invoice for the last ten months "rental" of YOUR modem with a note suggesting they can credit the cost of the new modem against it.
