[roughbeast]

Interesting.

I am now with Vodafone/CityFibre. Part of their service is 4G backup. Instead of their hub I use my own dual WAN router with VF's 4G Sim dongle on USB. We have had one short outage in 18 months. 4G came in seamlessly at 45Mb. Like you I only noticed because speeds were slower, but when I logged onto my Asus router it went directly to it, not to the the VF Hub GUI. This is probably because the VF backup is integral to my ASUS set up.

The only cause for your issue that I can think of is that you have somehow left your VF mobile in WiFi Hotspot mode and that your Hub 4 and other devices are using it as WAN automatically connecting to it using connection credentials used on a past occasion. (Hub 4 connecting to WAN by WiFi? Is that a thing?) Check that your phone is not acting as a WiFi hotspot. If in doubt, shut it down and then reboot everything else. With no other WAN connections apart from VM surely they will reconfigure.