double charged billing

This year rain has been showing flames, I used to have a 4g sim, and worked fine since I took it in 2021, so from 6pm to 12 midnight It wouldn't work, it was even worse on weekends. i tried calling them but I was kept on hold for over 30 minutes until I hung up. i decided to cancel the 4g , to my surprise I received a call from them the following day to enquire why I canceled, I explained my situation and they told me about the 5g, I asked to be upgraded to it. The consultant told me I was required to pay about R35 to use it for the month, and there were no other outstanding. i paid R600 around the 25th of March so it will cover April, to mu surprise I was billed R17000. i don't even know how it came about. the consultant called me to remind of the payment but told her I won't pay unless they explained the invoice, she told me she has escalated to her supervisor but haven't heard anything.