Bad customer service and repair

I experienced the worst customer service from the cell c store in secunda mall, I purchased a ZTE mf668 from them and when I got home installed the software and tried to connect it gave me an error message saying I must contact the customer service, I called the call centre they told me to take it back to the cell c store I bought it from for a software upgrade the following day I took I back they said they'll book it in for software upgrades in joburg and that they'll sms when its back. I had to go back several times to go check if its back because there was no communication from their side which made me grow extremely frustrated, after about 4 weeks I went there to collect it after no sms or call from them, went back home tried using it and still it was the same error message, extremely furious I went back demanded a new modem and they refused saying it must be booked in again and after 3 book ins would I be able to get a new one I don't get how they can sell me a faulty device and still want to book it in 3 times which takes ages to come back and I paid R1600 for it.