No cellular service

I have renewed my contract 08/2014. Since the beginning my contacts cannot get hold of me. I thought it was the sim so I did a Sim swap. Still had intermmittend service. With my network mode selected to WCDMA/GSM or WCDMA only, the phone displays that no service is available. If I select GSM only I can use it but only in selected area's. I used my wifes Vodacom sim and I have 3G wherever I go. The service provider says I must take it up with Cell C. Cell C sends me little messages telling me to play with network settings. In the mean time I have a Galaxy S5 on an expensive contract and I can can only take photos with it. My wife and I travel a lot and it is becoming a security risk for me. Surely I should get help from somewhere. I cannot continue to pay a lot of money for nothing. What should I do?