You are on a business account, you have SLA's and a business advisor. Get on the phone, tell THEM to sort it out and sit back and watch them finally work out its the router. We've seen similar issues with HUB3
For balance, yes another router and modem mode would work, its NOTHING to do with static IP's. That sounds suspiciously like offshore supports kind of thing....
In fact if you have any trade at all, a business class device, or a proper install of something like a couple of UBIQUITI AP's would pay dividends. Most people who are using public Wifi wont give a damn what you charge for a coffee if the WIFi is good.
AFAIK the access point range do not have a DHCP facility. There is an option in the software but that is for ones connected to a UAC router.
Perfectly possible to daisy chain devices- Hitron in modem mode> cheap and cheerful router doing DHCP>UAC AP's - but its a bit of a fudge when the setup SHOULD just work. If you have a spare router lying round that should get you up and running whilst you light a fire under your Virgin Media Business Advisor.