I've seen a handful of reports of users with the Hub 4 saying that their hub gets full speed in router mode (or measured using the built in SamKnows facility) yet the output to a third party router is constrained to 150-290 Mbps, and I've yet to see a convincing explanation. It isn't ethernet cable issues, because if the cable were faulty or misrecognised then the hub would drop back to the very obvious 100 Mbps of a 10/100 standard. And I've seen it reported for premium routers, so I'm inclined to doubt that the MW6 is faulty.
I can't offer an explanation; One option to try would be to have the hub in router mode with wifi signals turned off, and the MW6 in access point mode, see how that performs. Normally it is better to allow a mesh system to act as a router, in this case that doesn't seem to be an option for you.
An alternative for wifi coverage would be to ask for one or more VM "pods", but I don't think they have an ethernet port.
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log into hub3 and hub4- 192.168.0.1 [or 192.168.100.1 if in modem mode] - DON'T SIGN IN, click - click router status [centre of screen] - and post the downstream and upstream figures - to see those press on the grey buttons/boxes labelled - upstream - downstream - also post the config details from the hub to be sure you have actually been upgraded to 1 gig
had you changed anything on the hub3 - split the bands and turned off smart wifi - if you had have you also done that on the hub4