Turkcell's Poor Internet Speed, No Support, and High Cancellation Fee

Today, at approximately 3 o'clock, Superbox was installed. Ever since the installation, our internet speed has never gone beyond 1 megabyte. The installer assured me that the issue would be resolved in 10-15 minutes before departing. Has the issue been resolved? Actually, it didn't. After making a call to the call centre, they were unable to resolve the issue. I made around 8-10 phone calls. Then I decided to cancel. They insisted on charging a ₺1500 fee for cancellation. It's only been a few hours since the installation, and their assistance has been completely unhelpful. They lack any form of assistance and show no concern. I have notified the Consumer Arbitration Board and my lawyer regarding the situation.