number of events cause the reservations to change and observed them over the time from logical subnet changes of course to maintenance and device being without power although this needed to be 24 hours plus from memory.

The support conversation thing was more my IT OCD and understanding how things work. If they said semi permanent DHCP reservation I would have been fine but they stuck by static. Could never get a full picture on their renewal process and reclamation. Seemed different than the device lease time . DDNS for "static" IPs it was then

Support email over via client to virgin so will see what they come back with.

It seems like it may be different now on how the Static IPs are handled as had seen a GRE Tunnel section and a /32 static IP address mentioned. So I am hoping they are static now.

Thanks for the search and I think it is difficult as there is a lot of noise from residential devices on the VM forums. At least that Ireland post suggests that tehre is some variance and support able to change things. Annoyingly they have the instructions for changing to modem mod eon their site but its not there https://www.virginmediabusiness.co.uk/help-and-advice/products-and-services/hitron-router-guide/dynamic-modem-only-mode-user-guide/

cheers for the help.

small update that virgin support said it would become available when the WIFI was turned off using button on front. Did not work so back to support I go.

Also trying the bandwith routing for VOIP phones over it with Force routing rather than Force NAT to see how it behaves.

For anyone else that ends up here the answer is relatively simple. You cannot have the device in modem mode with a single static IP. In Multi Nat setups with 5 IPs etc it is always in modem mode and you need an internal firewall.

on a dynamic you may get the choice but you can have it in modem mode at least.