Terrible experience

I took a router contract with Telkom 2020 after a year their call centre called me for upgrade and I was told to throw away the sim card since they were going to give me a new one, I did that but they charged me for that SIM card. I did the upgrade 3 times and I was told the same thing until last year April when I noticed that my account was debited double the amount we agreed on, I sent email but there was no reply or call from Telkom, I finally went to Telkom branch and the lady said she will stop the other 3 SIM card. I then asked them to refund me money that I was charged from 2021 for the other sim cards but they told me to go open a case, for something that they can prove and they could see, the following month that was May 2024 still they debited more money that we didn't agree on. When I tried to cancel the contract I was told I can't since I'm owing. Owing how when they owe me more money?