Internet speed wasn't good, it has been restored. Can I get a refund?

Yesterday a KPN Monteur came to my house and explained that I had been paying for 200 internet speed when really in my home I only had only had access to 100 internet speed. When I call KPN -grateful that now my internet speed does work-but with a request for my past bills to be rectified, I am told that if I was paying for more than what I was receiving this was indeed my fault. How can I have this reviewed and corrected?