Urgent matter with MTN

On the 5th of June I have app**** for an uncapped home router of R265,00 per month. I have been told that before delivery, I must pay two months in advance and Thabiso(the consultant) told me my next payment will be in August. I made of R502,14 through my Capitec app. On our telephonically conversation, I made it clear that I don't want any surprises. On the 3rd of July I received an invoice of R654,56 that is due on the 06 July 2024. I need help in this, before they black mail me, I know cancelling must be made a month before, if they cant rectify this, I would like to cancel with immediate effects and get my money back.