We're trained to support residential customers here on the community, so I'm sorry that I am unsure on how to help you. However, our Virgin Media Business teams are ready to assist, just give them a quick call or browse the help articles on the support page here. I'll also leave your thread open for other community members to add any info they may have.
Thanks again for posting,
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If anyone else stumbles across this post in the future, I'd had an answer but not the answer I wanted.
The VMB routers can't provide Modem Mode if you have a static IP, this feature is only available if you have a dynamic IP. Bazaar, but that seems to be the way it is....
Its Due to the GRE tunnel used to connect to the VMB static IP service.
I'll explain that in greater detail for anyone who is interested.
The Hitron, in Router Mode, uses a dynamic IP address in the same way as do the residential hubs (e.g. Hub 3). That IP address is fairly sticky and changes only if they move you onto another server at the local hub, or if you switch the Hitron off for more than a week (sort of thing).
Once the Hitron has connected with the VM end, it behaves like a client (such as a PC) and establishes a VPN tunnel back to the Virgin Media Business server (somewhere). Then, recognising your Hitron, the tunnel is routed through one of the static public IP addresses assigned to your account. Your static public IP address terminates at the VMB server not at your Hitron. This is the trick that allows VMB to operate on the residential infrastructure.
The VMB server translates all addressed data back through the tunnel to your Hitron. In modem mode, all that clever stuff is shut out. I read somewhere (I think it was in cableforum.co.uk) that there was a way of using a high grade router to acquire the GRE tunnel back to the VMB server using the router's VPN functions; just not 100% sure.
I have today requested to be set back to dynamic ip. which has been done. Where you can then turn off DHCP which says you will be entering modem only mode. How's ever I do not believe this is true MODEM MODE. it's just not dishing out ip's and closed down some ports. and you will still get conflict problems.
No DOCSIS gateway I know of is "true" modem in this sense (probably because you never want to spew full DOCSIS datastream to user-accessible port). All of them are routers (L3 devices), implementing "modem mode" as half-bridge DNAT (replicating user IP to one of ports and disabling others). Since in this mode they still function as routers, they are still sometimes prone to mishaps (e.g. closed ports or issues with some more exotic IP packets like L2TP).