There's plenty can be done, but phoning VM is a waste of time - they're apparently proud to have the world's worst telephone support.
In answer the overall question, the minimum speed guarantee applies only to a device connected via an ethernet cable, because there's a range of variables outside VM's control regarding wireless performance. To get things sorted, we can talk you through various checks, but if the contract is not in a fixed term (discount) deal, then bear in mind that changing ISP may be an option. VM's higher speeds count for nothing if they're not available consistently and reliably.
First of all, I'd like you to do the following, ideally after the hub has been running for 24 or 48 hours since the last restart. Connect to the hub by clicking on this link http://192.168.0.1/ That should pull up the log in page for the hub. But don't log in, just click on the link "Check router status" That'll bring up a window with five tabs. Open the Downstream tab. Select all the text (Ctrl-A if using a keyboard), copy it (Ctrl-C), then paste it (Ctrl-V) into a reply here as TEXT not screenshots. Post that, do the same for the Upstream and Network log. You'll get an error message when you post the Network log, just click on "post" a second time.
From that we can check for any obvious problems with power, noise or error counts on the broadband connection - although that's not necessarily the root of the problem, it's a straightforward check. If that's all good, then we can assume the problem is in fact a wireless issue, and start asking questions on where the hub is, type of property and construction of internal walls, whether you'd spend money to improve the wifi and so on.