MTN Contract Urgent Issue - Please help

I took my first contract with MTN in 2018. And i had no issues. In 2019 i then took an additional contract pay R699 monthly. When the Covid-19 pandemic started, i was offered a relief plan. so my 699 contract and i was left with 12 months. I was then told that they would decrease my monthly payment to R350, and ofcourse add another 12 months to my contract, which made sense. The first month was billed properly. Was i not shocked on the 2nd month when suddenly i was billed R467.00, when i called, customer service couldn't assist, and told me to send an email, i did, and there was no response. I kept calling and receiving no assistance on being billed properly so i can pay, because i refuse to pay an incorrect invoice. I then advised them and said if they struggling to bill me properly, they must take me back to my usual R699 monthly and take me out of this relief because they clearly not doing what they said they would when billing. I have not been assisted, and after months of refusal to pay incorrect invoices, i have been handed over as if i dont want to pay when all i asked for was either bill me correctly the R350 promised or take me back to my original R699.00. Please urgently assist, they do this because they know it's my credit record that's being messed up.