You'll only see a maximum of about 600 Mbps over wifi because of the technology that the Hub 4 and BT disks use (Wifi 5), and if the signal is attenuated by thick walls or the device itself has lower capabilities speeds will be well below that, even with a mesh wifi setup. A Hub 5 (still on trial, so not rated for prime time release) might give you faster speeds close to the hub thanks to Wifi 6 if your devices support that, but beyond that no, because the BT disks are still Wifi 5 and the range of the Hub 5 is not materially different to other VM hubs. Even if you got a Hub 5 and VM's pods to extend it, those pods are still Wifi 5 so the same speeds as BT disks are likely.
VM technicians never check wifi coverage - it isn't priced into the contract which assumes a quick install-and-run and as far as VM are concerned wifi coverage is the customer's concern.
I assume that you've looked in the hub's settings and checked that the hub is showing full speed via the built in SamKnows test? If that's indicating 1 Gbps speeds, then the hub is performing as expected and your problems are indeed with wifi. If that's only showing say 350-600, then there's something wrong with the broadband side of things. That would be (probably) fixable, but you'd still not see above 600 Mbps over any Wifi 5 connection.
If you're not happy then yes, you have a 14 day cooling off period starting from when the connection went live, invoking that will take you back to whatever arrangement you previously had with VM in terms of package and speed.
