The hub isn't able to put into modem mode because I've got a static IP (I've double-checked this reality with Virgin tech support, and they confirm that it's true).
Strangely when I was performing the setup of the Deco X50 devices using the TP-link Deco app, I could only seem to get Internet access (when connected to the Deco wi-fi) when I told the app that I had a dynamic IP. When I told the app that I had a static IP, I couldn't get Internet access.
I'm not sure if that's correct or not. It's possible I wasn't getting Internet access (when I selected the Static IP option in the TP-link deco app) because I wasn't waiting long enough, and/or because I wasn't entering the correct settings (IP, subnet, DNS, etc.) for a Static IP.
But I'm 99% sure I was entering the correct Static IP settings.
So maybe I wasn't patient enough. I don't know.
Does anyone else?
Hang on, are you on a normal domestic connection or a business one? I’m guessing that it’s the former as you have a Hub 5. In that case you absolutely do not have a static address as VM don’t offer them on domestic connections, you almost certainly have a dynamic public IP address!
If, indeed you are on a normal domestic connection then you haven’t spoken to VM technical support (assuming that such an entity exists), you have actually been speaking to a random call centre worker on whatever passes for minimum wage in that part of the world, who probably has never even seen a VM hub in the flesh, so to speak, and whose training consists of reading from a pre-scripted set of responses in front of them!