shocking service and online contact numbers don't work -how do you fix a problem - MTN doesn't!!!!

I used my new bought MTN locked cell a month ago. Used and worked well. The suddenly says sim not supported. I go ins**** (the opposite of service with a smile). I seem to have to RICA the sim. Redone. Working. The morning after again sim not supported with additional note that may not be an MTN sim!!!! Cannot contact MTN as cannot use the cell and cannot call numbers on website - message says they don't exist! Real time online chat I got a simple sorry about your sim and the person ended the chat. MTN poor form poorest service.