Turkcell Postpaid Line Billing Issue After Promised Monthly Price at Signup

I visited the Turkcell Gaziantep Gazikent Burak Mahallesi Özdemir branch to obtain a new postpaid mobile line. I was explicitly assured that my monthly bill would not exceed ₺450 and that there would be no additional charges. I was persuaded to sign up based on this guarantee. However, my first bill was approximately ₺752, and my second bill reached ₺1,750. I feel clearly misled because these amounts are far higher than the price I was promised. The last four digits of my registered phone number are 2765. I contacted customer service and revisited the branch where I completed the transaction, but neither provided a solution, corrected my bills, or offered meaningful assistance. Since these charges contradict the information and assurances given to me when I signed up, I request a thorough review of the bills for the relevant periods, cancellation of any unjustified charges, and adjustment of my bills to the promised monthly amount of ₺450. I also request a written explanation regarding the misleading information provided to me and a proper investigation to prevent similar issues from happening again.