ADSL problem logged 11th Dec 2017 - still waiting

Paying for 20Mbps ADSL service for more than 3 years but getting only 10Mbps. Was promised a refund but still waiting. Logged a call on 11th December 2017 for a problem with ADSL cable and is still waiting for it to be fixed. Have phoned call center numerous times, have chatted on app, have send e-mails - what more to do? Worst service ever.