So I've figured this out but still have no idea how it happened.

I was using the TP-Link Tether app on my Android phone when everything lost internet connection. And then the Tether app refused to find my VR2800 again. So today I uninstalled Tether and reinstalled it via the cellular network, logged in and it found my VR2800 straight away.

Tether only showed one connected client (my main PC on Ethernet) and when I clicked on the client button it showed 30 something "blocked" clients. All of my other devices that suddenly lost internet yesterday!

One by one I unblocked them and everything is back to normal. As to how Tether managed to instantly block every device on my network except my main PC is a total mystery to me though.

Think I'll be deleting the Tether app for good measure.

  @David-TP  In the Tether app you can press on any individual connected device which will bring up it's name, connection type, IP address and MAC address and there's a Block button at the bottom of that screen or if you long press on the device first it brings up the block option without going to the details page. I can't see any way of blocking or unblocking multiple devices at once.

I can't be 100% certain that it was the Tether app that caused the blocks but very coincidental that I was looking through the app at the time that it happened. And this has never happened to me previously (been running the VR2800 for a bit over a year now).

Interestingly the day after I figured out what had happened it did it again but this time it only blocked 4 of my device connections. I did look in the router log files but couldn't see anything in there to indicate the blocking activity.

Also, it wasn't just my mobile devices. At first I thought it was just my wifi connections but both my TV and a Raspberry Pi are connected via Ethernet and both of those were blocked too. Only my main PC on it's Ethernet connection wasn't affected (unplugged it's Ethernet to force it to wifi and no internet).