Not receiving contracted speeds

I have been a Vox subscriber for several years - first on ADSL, then onto fibre. My initial package for for 20Mbps worked well. Then I saw that they were advertising 40Mbps for less than the 20Mbps and upgraded to the higher bandwidth - I don't think it's particularly ethical not to adjust month-to-month contracts to new rates or that they don't inform their customers of this saving. Now on the 40Mbps package, I get download speeds as low as 2 with an average of well below 20. Several calls to their support where they 'reset the port', etc. resulted in short lived speeds of 35Mbps , but it keeps dropping down to below 20 again. I reckon that they are oversubscribing their customers and that they cannot support the speeds that they advertise.