mtn wrong billing of lenovo AMD with ip5

ive been complaining to mtn about a wrong billing from june 2019,i took laptop(AMD LENOVO) accout of R199 per month and they are debiting R434 monthly.i went to the 1000 times & they always say its fixed,i even spoke to the manager and he promised to fix the acc 4months back but he is failing,i even wrote an email to mtn sa & they requested my aac number as always but nothing is happening.i took the acc @mtm store mall of the north limpopo,please mtn improve your service & fix my contract.