Faulty Wifi Modem

I was called by Vodacom and offered a data contract which included a Vodacom Wifi Modem. After using the Modem for about 5 months it stopped working. I took it to the Vodacom repair centre at Tygervalley Centre and booked it in. After 7 days I received a new modem. After about 4-5 month's I was back at the repair centre in Tygervalley centre with the same problem on the wifi modem. I was told by the consultant in the store that my specific make of modem is problematic. Customers are always bringing in the modem as it stops working. I complained to the Service manager that I'm not happy with this faulty product that Vodacom offered me, locked in a 2 year contract. The manager apparently escalated my query and 2 weeks later I received a brand new modem. But the same faulty brand of modem. This is now the 6th time that the wifi modem I receive from Vodacom has stopped working. This is is unfair business practice. I now have a Vodacom Wifi Modem, with data on it and I cannot use it because once again, after 4-5 months it stopped working. Vodacom, why did you offer me a contract with a faulty product? Should I just hand it in again at the Repair Centre? What are my options???