I have just renewed my Virgin Media contract and they sent me a new Hub 5 as part of the deal.
My old hub was in modem only mode, so I thought it would be easy to swap them out.
However, my router was failing to connect to it, saying “Not connected”. I tried many times but I could just not get it working. I could connect to the internet directly from the hub, but not via the router. I found a note about the 5x hub (which mine isn’t) that suggested taking the hub out of modem only mode and in the settings disabling UPNP, DMZ and DHCP, before switching back to modem mode.
I followed these instructions and sure enough, when I switched back to modem only mode my router connected immediately.
Sigh.