So I have had an upgrade made to GFast by TalkTalk and an OR engineer came to set it up as well as giving me a white modem which is due to be used into TT's router WAN port. It worked for like 2 hours before dropping completely and has never worked since despite trying different solutions. TalkTalk is obviously incompetent and unable to have a proper technical conversation, so it's hopeless with them. They are sending another engineer but we all know nothing is gonna change.
The engineer could test the line at the master socket fine and had the router connecting to internet after a call with DCoE. Not sure what was said then. I saw the connection myself on his device so I know that everything is ok to the socket.
The GFast modem has all green lights on and seem fine.
I suspect TalkTalk authentication to be at fault but they won't help and their router hides all the settings I'd need to see.
My solution, then, was to first assess if I can connect to internet with my laptop directly plugged into the openreach modem and make a dial up connection with BT standard login details. For some reason though, I can't connect and get the error 651. I tried no login at all and same. Shouldn't I be able to connect to it straight from my PC? That's very odd.