Paying for something that I am not getting.... Breach of agreement

We had a fault logged on 21 April 2018 for slow internet speed. The fault was closed on 03 May 2018 by the technician. I spoke to the call center, to be informed that the fault was closed, as the technician was on site and it was determined that there is congestion on the line. The previous time when we had the same issue, the technician advised that the box and cable would have to be replaced due to corrosion on both the box and the line. I am paying for a 4mb line and I am not even getting to 1mb! You are in breach of agreement....