UNABLE TO REPAIR HEADSET/UNABLE TO REPLACE HEADSET

NO LTE COVERAGE ON SAMSUNG GALAXY S6, PHONE HAS BEEN IN FOR REPAIRS TO POLOKWANE VODACARE TWICE. THE FIRST TIME A FIRMWARE UPGRADE WAS DONE, NEXT NO FAULT WAS FOUND. I DID A SIM SWOP, AND THE PROBLEM PERSISTS. VODACOM WANTS ME TO BOOK THE PHONE IN TO ADVANCED REPAIR CENTRE, SAMSUNG SAYS THE IMEI NR NEEDS TO BE REWRITTEN. I NEED A PHONE WHICH IS WORKING, WHY DO I PAY MY CONTRACT EVERY MONTH. I LOGGED A COMPLAINT EARLIER OF WHICH NOTHING CAME. I AM REQUESTING A NEW PHONE, IN A WORKING CONDITION.