[Summary: you don't need to put the Super Hub into modem mode. Just connect the Archer and set that into access point mode.]
We've always had issues with our Virgin Super Hub 2. Originally the only problem was that Amazon Kindles wouldn't be able to reconnect to WiFi from airplane mode unless you rebooted them and the router at the same time. Later we found it was useful to reboot the Super Hub occasionally to improve WiFi speeds. But as the number of wirelessly connected devices in our home rose to over twenty, things really went downhill. Now my PC fails to connect to the WiFi when my son (and his devices, presumably) is in the house, and the WiFi vanishes completely for a few seconds whenever it feels like it.
Theoretically you can ask Virgin for an upgrade to a Super Hub 3. However, if this feature is available on their website, it's so well hidden it might as well not be. I could try ringing Virgin, but previous phone conversations with Internet providers still haunt my dreams, and in any case, for all I know the Super Hub 3 will be just as bad.
I decided instead to buy a new router entirely and connect it to the Virgin one. The idea is that you put your Super Hub into "modem mode", and then plug the new router, in my case a TP-Link Archer C7 (AC1750), into it by ethernet cable. Virgin even provide instructions, which look simple enough.
I picked up the new router for about £65 from Currys. The Archer C7 had got good reviews, sounded like it could handle the number of devices you find in a house nowadays, and I couldn't see how it could be much worse than what we have now. It arrived in the post a few days later (I'm writing in the days of the (first?) Covid-19 quarantine, so going to collect it wasn't an option), and in its instructions I saw the part about "access point mode": how to use your new router to extend your existing network.
So I followed the instructions for the TP-Link, then switched the Virgin hub over to modem mode, and waited. A few minutes later the Super Hub 2 was showing three lights: Power On, Ready, and Internet. The TP-Link was showing four: Power, 2G, 5G, and Internet. Sadly the last of these was in orange, meaning that there was no internet. Well, there's a surprise! Fancy things not just working straightaway.
I rebooted both devices, but they stubbornly refused to cooperate. As switching off the internet in this house for longer than 30 minutes is no joke, I did a factory reset on the Virgin box, switched off the Archer C7, and left it at that for the night.
Google searches the next day didn't enlighten me much. The most promising suggestion was that I hadn't left the machines long enough to "settle down", so all I could do was try again. I started by plugging in the Archer C7 again and switching it on. To my surprise, it connected with the Virgin box immediately. And I hadn't even put the Super Hub 2 into modem mode!
It's been running fine for two days now. Our WiFi speed is substantially faster, everything connects to WiFi automatically, and nothing has lost contact either. To reduce radio interference (with our neighbours, if not with ourselves), I've swapped all our devices to the new WiFi, and disabled the Virgin WiFi. Looking good.