Over charged monthly

I bought 2 cell phones deal that was coming with smart watch for free as they sold to me saying its a promotion. To my surprised I have been charged for 3 devices. I have been calling MTN as this is affecting me financial. I was told it's fixed but this is not true because I just got the billing invoice and I am still overcharged. I was promised to be credit for the previous 2 months that is also not happening.

Can someone please intervene and get this matter sorte