Just so other people know.
To solve the problem, I downloaded the ZTE modem update from Cell C site. I then, through trial and error, unistalled and reinstalled the driver. It's a miracle! I managed to connect to the internet. I did this without the assistance of the Cell C people who failed to help me . (I don't think they know what they are doing.) However, the connection is still (as usual) slower than a sloth with parasitic fungi growing on it's toes.