@Jack111 

Hi, "Hub is in modem mode", did you set the new VM hub 4 as a bridge modem?

How about the internet connection type? Have you received any PPPOE username and password from Virgin Media?

I think you could refer to the following post to find the correct internet configuration details:

Recommended Connection Types for Major ISPs in the UK

PS: if you want to completly remove the old system, please hold the bottom reset pinhole for 1s only on the Deco to run a factory reset first.

Thank you very much and best regards.