Superonline Charged Modem Fee Despite Device Being Returned And Not Recorded

I am a Turkcell home internet subscriber with customer number 75412***. I returned the modem belonging to this subscription by cargo to a Turkcell dealer in Izmir on October 17. Despite having delivered the device, I received a message on November 2 stating that a modem fee of ₺1,322 had been reflected on my invoice. After noticing this, I contacted Turkcell customer service and explained that I had already returned the modem. However, I was told that they could only deposit ₺2.06, and no solution was offered regarding the remaining amount. Although I fulfilled my obligation to return the device, I am still being charged as if the modem had not been delivered. I request that the modem fee of ₺1,322, which was invoiced despite the return of the device, be fully refunded to me and that the records be corrected accordingly.