Very annoyed :(

I have had a few contracts with Cell C over the last 5 years, and have never been more annoyed with any other service provider. I recently upgraded from my Samsung S4 to a Samsung S6, and have not been able to use it since. The phone means nothing when you NEVER have any signal or network. I cannot make a phone call when needed and cannot receive calls as my phone is constantly on VM. I am beyond the point of fed up. I have contacted Cell C call centre, and the consultant advised that I switch my Network Operators setting to Vodacom - how pathetic is that!!!! I did however change the setting as advised - but no luck! This needs to be fixed as soon as possible...