The Virgin Media support forum I find is far more useful than the proper Virgin Media support process.
So I posted there....
Original problems as posted to Virgin Media support forum
I have my VM router in modem mode and use my own router (ROG-Rapture-GT-AC5300) to manage my firewall and internal networking needs.
When I originally jumped through hoops to get this setup working (after spending ages with VM support, including paid support, without success) I ended up connecting a computer directly to the VM modem via ethernet patch cable, allowing the connection to become established, then cloning the laptop MAC address into the router.
Every now and then the modem/router lose the connection to the VM network. To fix that I have to plugin a computer via patch cable to VM modem again to establish the connection.
I can then unplug the computer and the network connectivity will resume service as normal, even while leaving the MAC address the same in the router.
I'm assuming this is *something* to do with VM security stuff forcing something to reconnect that I guess is working in a way that is incompatible with the router (or the current router settings).
Firmware updateAnother failure of my Virgin Media cable internet connection and my Asus router (Julr? 2020) to behave properly led me to another session of Googling for networking advice, trying to track down potential issues in compatability between Virgin Media and routers, specifically this Asus router.
Eventually I tracked down a post in a forum somewhere that looked a bit like the problem I was having. The poster there claimed to be able to set the WAN DHCP mode to 'continuous' and that sorted their problem.
I had no such option at the time and had tried all options I could find about WAN DHCP as I already had suspicions about that area of functionality.
So I tried a firmware update, I do generally keep software and firmware of my devices up to date for exactly this sort of reason.
Turns out there was an updated firmware waiting to be installed, and afterwards I gained a continuous option in my DHCP settings.
So far so good, my ASUS Rapture AC5300 seems to be working better with Virgin Media network now. Fingers crossed it continues that way.
So there you have it, if you have an Asus router and issues with Virgin Media cable connectivity, try WAN -> DHCP Query Frequency -> Continuous.
If need be, try updating your router firmware.