Sorry but I honestly don't know what to suggest if you have carried out my suggestion. My SmartHub2 and Disc behave in a totally different way to yours, the RJ45 LAN ethernet connection is used by default if connectivity is present, if not (CAT6 unplugged) it will switch to WiFi.
My conclusion although probably wrong is maybe the physical LAN port is faulty on the DISC and/or the router? What happens if you have the DISC connected to the hub via WiFi and you connect a device (turning off wifi on that device) to the DISC's ethernet socket? Does that device then have internet connectivity via the ethernet connection, or not and if so is it stable?
Another question are you looking at the how the disc in connected via 'advanced settings' - 'wifi' - then discs and expanding the information there? If so can you post a screenshot?
It will be a call in to BT's helpdesk, see what they suggest, I don't know the number.
Thank you for your continuing efforts. I think the next step is to get ahold of another disk and test it.
I have just attached a different disk which has connected via ethernet. I will check the connection in 24hr and let you know if it changes