This will need an engineer visit to sort out the power levels. You can call VM and try to book a visit or wait for the VM staff to get to this thread and they can help you book a visit. Unfortunately current response times on here from staff are about a week.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong If my answer solves your problem please mark it as helpful as it may help others My setup: Vivid 200 Optical fibre with Superhub 2 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.
Thanks for taking the time to post on the community.
Sorry to read that you are experiencing issues with your broadband connection.
When I checked your account, I can see that the power levels are running at +11 dBmV as you mentioned. For this I can arrange an engineer appointment for you. I've sent you a PM (purple envelope icon, right corner) requesting some details for you to confirm please. Once you have replied I can go ahead with the appointment.