If the problem is happening on all your devices and on all the different hubs and routers that you have tried it would appear that the problem is not with your equipment or settings and must lie somewhere on the BT network.
I think you have done all you could reasonably do to try and identify the problem without success.
With the above in mind I have notified the moderators of the forum about your problem. Once they have read this they may be able to help. They are a BT UK based team and if they can help they will reply via this thread asking you to contact them via a link. They are very busy at present so once you have replied to them by the link, it can take 48 hours for them to re-contact you.
It'd be best if you spoke with one of us so we can help you get to the bottom of it. It may take 1-2 working days for us to get back to you. To reach us, click on my user name and then on contact the mods. Please make sure you do this whilst logged in and also using our full site view rather than a mobile view.
Still no reply from BT...i have found old open reach modem, I conneceted to the Netgear router and is working everything fine (no IPv6 btw. D6400 is not compatible with BT IPv6)...i know there is a smart hub with WAN connection (one I have it doesn't have one) Does anybody know how I can get one, please?
Still no reply from BT..
Under ideal conditions, the moderators can normally deal within 48 hours, but they are running a bit behind at the moment as they are only a small team based in the UK. They will however take full ownership until any issues are resolved. They are a very good team.
I assume you did fill in the request form?
As the Netgear router is not using IPv6 and is now working, then I still believe that either IPv6 is not setup correctly on your devices or there is something wrong with the IPv6 on the smarthub.