Oh to reply to myself in case it's useful for anyone else - I was accessing my router using a hostname that directs to my home network's internet-facing static IP address. I changed to entering the...

Did you ever get to the bottom of this? I have this exact problem with my tp-link router too!

I've made a diagram to help with understanding the problem - hope it's useful. Computers 1, 2 and 3 can all see each other but cannot see Computers A, B or C.

To add to my own post, I have just tried swapping out the switch for an old switch I had which was slower (10/100 rather than 1000Mbps) but otherwise working before I replaced it. I still have the...

I have more wired ethernet devices than there are ports on my Archer VR2800 so I have used it with a switch connected to one of the VR2800 ports, and the surplus devices connected to the switch. This...