Turkcell Fails to Provide Signal and Internet on My iPhone 15 Pro

I live in Pendik, Istanbul, in a detached house located on the coastal road in a central area with 5-star hotels just 100-200 meters away. Despite this, my Turkcell mobile line barely gets 1 bar inside the house and 2 bars in the garden. At times, I am completely unreachable inside the house, and conversations are frequently interrupted. My phone is an iPhone 15 Pro, and I even tested the sim card in a different iPhone 16 Pro, trying various internet settings, but the problem persists. Internet access is nearly impossible. Turkcell's service is non-functional in this central Istanbul location, while other operators have full coverage. I reported the issue to customer service, and they informed me there might be a transmitter problem in the area, promising to check within 12 business days. However, no solution has been provided yet. Turkcell once advertised its 4/4 coverage in caves in Anatolia, but here we are approaching 2025, and their service doesn’t work in the heart of Istanbul. For reference, see their Damlataş Cave advertisement. I demand an urgent and definitive solution to this issue—I expect proper service for the ₺3,000-4,000 monthly bill I pay!