The thing that immediately comes to mind is a faulty hub, incidentally what did you upgrade from? It may be that the hub was always faulty in modem mode but you just didn’t notice it in the past because your rated speed was less than what it could cope with anyway.
Have you tried the good old pinhole, full factory reset, just in case the firmware is being a bit troublesome and needs a kick?
Other than that, the only other thing would be a hub replacement, which you might get VM to struggle to understand why, ‘but it works in our preferred router mode, so where’s the issue?’
I’m slightly surprised to hear you say that 1 Gig isn’t available in your area? Who said that, by the way, as I’m sure that VM made a big fuss of claiming that it was available in all areas now and all our customers can benefit…. .?