Unfair business practice

I would like to lodge a complain against Telkom mobile. I have a fixed 24 contract with them which expires in September 2021. The agreement was to pay R750 monthly and if i have consumed what is allocated for me i topped up. During the month of March 2021 they send me a bill of R1700, when i query it they told me i have upgraded it and they cannot cancel it as i have to do it on my cellphone. The phone i used by my kid. I paid that R1700 on the 31 March 2021. To my surprise, today, 05 April 2021 i received a bill amounting to R1900 to be paid by end April 2021. Please assist me as i am being ****** of my hard earnings by this company. Your assistance on this matter will be highly appreciated as i cannot pay such an amount which i didn't agreed upon. Regards, SF Yona .