Why Did Turkcell Change My Plan After Buying an Additional 5 GB?

When my Turkcell internet package expired, I went to the add-ons section and bought a 1-month 5 GB option. After waiting for it to activate and it did not, I contacted Turkcell. The customer service representative then informed me that my plan had been changed as a result of this add-on purchase, and they refused to cancel when I requested it. Despite explicitly stating that it was a one-month add-on, they insisted on keeping it. I've been with Turkcell for seven months, with five months remaining on my 12-month plan. Previously, I paid ₺280 for 20 GB and 1000 minutes, but now they want ₺280 for only 5 GB, with additional monthly charges for plan changes. Selling the main package in the add-on category is an obvious Turkcell app flaw. If cancellation is not possible, please contact me to switch to a bill-free plan or cancel.