When 20Mbs is only 8Mbs

I have a data contract with Telkom but with loadshedding their backup power does not last more than 40 minutes, so although my business has a generator, we cant work as we work mainly online. I went to Telkom and was told if I took a 20MB DSL line there would not be any problems. I had to pay a contractor to lay conduit for Telkom's lines and after a long delay the line was installed. I asked the technician to test the line and it runs at 8Mbs instead of 20Mbs. He said there is nothing better than 10Mbs in my area, so I went to cancel the contract with Telkom. We are now over a month on, and I am being debited for a service I never used and will never use as its to slow. I have sent 3 emails to Telkom and been to their branch. What more can I do????