Turkcell Superonline Fiber Internet Speed Complaint

I am a Turkcell Superonline Fiber internet user. Since my 200 Mbps subscription's contract period ended, I upgraded to 1000 Mbps by paying the price difference during the renewal. However, since the upgrade, I have not seen any change in my internet speed. When I call customer service, they claim I’m getting 870 Mbps. We’ve tried restarting and resetting the modem, but I never see those speeds. I’ve attached a screenshot showing the maximum speed I can get, and here is the link to the test result: https://turkcellhiztesti.speedtestcustom.com/result/1c603a60-1889-11f0-ad51-fdc31dbd4f6b. They refuse to open a support ticket, saying there is no issue or fault. When I ask to speak to a supervisor, I’m told someone will call back within 3 hours—apparently, they’re extremely busy. They even offered to send technical support for a ₺600 fee, which feels like a joke. I refuse to pay for a service visit caused by their own internet issues. I want a technician to come free of charge, test the speed next to the modem, and show me that I’m getting 870 Mbps. I do not accept being told there’s no problem just because someone checked a screen at the call center. My request is clear, and I’ve already filed an official complaint through Presidential Communication Center.