Breach of contract!!!!

I upgraded my home Wi-Fi line, and I received an S90 plus 4G FWA then about 2 months down the line it stops working. I sent it in to Greenstone Vodacom and they said it's the motherboard and that they will need to get me a new one. I get a message saying they don't have stock of the one i received in the contract but that they have a different model and want me to pay in the difference!!!! This is absolutely unacceptable! They say they can't help me any further unless I Pay in!!!!! I want what I paid for in my contract and I can't even cancel it now?!!!!!!