Really disappointing service - who is accountable?

I would like to lodge a formal complaint against MTN. In a nutshell I have done a telephonic upgrade on 09/11/2020. Since then I have had endless problems, firstly with the SIM card, which did not work and was only activate around the 19th, 20th and once activated we can now see the DEVICE does not work, it does not accept ANY SIM cards.

I have dealt with numerous people over the last 3 weeks, so many different reference numbers and so many promises broken. One person even gave me their direct contact number, only to find out the number is not registered on the network. When I call I get transferred from one to the other. No one can assist...even been to a store twice. Yesterday I was on hold for 42 minutes with the contracts department before the phone got cut. I have asked for severel managers to call me, and nothing to date.

I am at my wits end. This phone is to be used for business and I am expected to pay each months for something that does not work. I have informed MTN on my intentions to lodge this formal complaint, and still nothing.....

Please help, urgently. Or advise where I need to escalate this to.