Any ideas as to why my BT Whole Home discs have stopped working?

[SueHoHo]

Hi all
I get my broadband from NowTV, but the wifi was weak upstairs, so I added two BT Whole Home discs (the white ones), which were easy to install (one connected directly to the router, the other by wifi) and they worked fine... until a few weeks ago.  

I did all the troubleshooting things suggested by the app and the helpline without success, so BT sent me two replacement discs, but I still have the same problem.  The main disc (connected by ethernet cable to the router) attempts to connect (purple, then blue flashing) but ends up with a failure-to-connect red light.  The router itself is working fine; NowTV have done all their checks and said 'not our problem'.  

I don't have anything else attached to my router, just one PC.
I don't have any other mesh system in my house.  
I have tried switching the discs.
I have tried switching the cables on the two ethernet ports on the modem.
I have tried restarting the router.
I have tried resetting the router.
I have tried resetting the discs.
I have checked that the router is on a dynamic IP address.
I'm using a new ethernet cable and refurbished disc from BT. 

I'm using the Whole Home Wi-Fi app (which is blissfully unaware that the main disc is not connecting), though connected to the NowTV wifi rather than BTWholeHome because, well, the latter isn't working!

I seem to have reached the end of the road with NowTV's and BT's helpdesks, with each thinking that the problem lies with the other's equipment.  

Any ideas?  All suggestions gratefully received. 

[Tablespider]

I have a similar issue... I have a set of three, which have been working fine until the last week or so, where the one in my bedroom, some 8-10m from the hub through light walls, has turned red.

I've unplugged both the nest and the router together and individually, I've changed  the actual discs over so I'm convinced it's not the disc, I've changed my plus to ensure nothing is leaking and interfering with the signal (but as I say, it's been working fine for the past 12 months..

I've pressed the WPS button, which helped on one occasion where it stayed blue for a day but turned red again.

I've replaced the ethernet cable which connects it to my TV but I can't see how that would affect the disc.

Any advice would be most welcome.

[SueHoHo]

Sorry to say I have made zero progress with my issue (partly because I am out of ideas, also because a family crisis has occurred).  I think my next step is to change broadband provider, in the (vain?) hope that the discs cooperate with a different modem.