Incorrect Invoice and no resolution / answers or customer service

I took a wifi contract with MTN advertised at R399 per month. I took this contract the last week of October 2023. I paid for the last week of October plus November month prorated to the vaue of R579.20 which is fine I know it is proated payment. 1 November I get an invoice for R888 for November and December. November is alread paid. You dont get any help from them they just say it is protated payment in bold letters without even answering or understanding what you are asking. Bunch of *****sters.