MTN lack of service

My husband has a contract for two phones with MTN. One phone was upgraded. New phone arrived with new 5G SIM card. Sim worked for a day then stopped. My husband went to MTN Howard Centre to have it sorted. He then phoned me from MTN to check if in fact it was working. It was. He came home and discovered that his sim was deactivated. I went to MTN and tried without success to sort out the problem. My husband went back to MTN Howard Centre yesterday and the same procedure happened. Both phones worked. He gets home and discovers his sim is not working. We’ve tried calling customer service but after several attempts we got through only to be cut off. My husband had to buy a new R150 SIM card each time. Does anyone have any idea on how we can fix this problem. MTN certainly have been useless.