I pay for 350 however wirelessly I get less than 20. It worked well for two weeks when I first go it. For the past 6 months it has been rubbish. Internet unstable and slow WiFi. What can I do in this situation.
With the ethernet connection being good you have a Wi-Fi problem that could be caused by one or more external factors.
TBH the easiest way to get round this kind of problem is to put the Hub into Modem Mode and use a third party Router / Mesh System / Booster.
You can approach VM to see if they will offer a free Booster or two but if they want to charge you £3 / month you would be better putting your money into something that will probably be better than the VM stuff and you can use it with any future internet providers.