I have done some reading and wanted to provide some clarification for other forum users regarding some of the references and comments the op made. There is an inference that Virgin Media are blocking third party routers from access to their network and only permitting the MAC addresses of shubs and thus making modem mode redundant. This is not the case. From reading the VM forums and looking at the particular issue it is all around dhcp (i.e. getting your ip from VM) requests and the frequency at which your router asks for them. Most of us know what pings and DDoS attacks are and that you can tell you router to deny ping requests. The same thing is happening here or something similar. Virgin Media are obviously in control of their shubs and how they operate. The frequency at the which the shubs requests dhcp info will be inline with the parameters VM set so VM know they won't be an issue. If however a third party router is constantly sending unnecessary requests then it looks like VM are denying the requests their end so the router doesn't get a new ip address. At the moment it looks like it only effects Asus routers and can easily be fixed by changing a setting in the gui. From reading the VM forums you would think there was a mass conspiracy underway but when you think about it it is a perfectly reasonable thing to do and I am sure the same process is applied in many other aspects of communication and maintenance requests over the network. This is the first time I have heard about it so I doubt it will affect many users however if you need further help you can have a look at this Asus support article for the setting to change. As I mentioned above, Asus routers are set to "aggressive" by default and I have never had a problem with it.
https://www.asus.com/support/FAQ/1043591/