Using extenders will definitely be an issue here and slow down your connection. Even expensive extenders will rarely give you more than 200mb...that’s if you’re lucky!
It's good that people offer help.
Without having an ethernet cable trailing across the room are there, to your knowledge, other options?
If not, perhaps BT should advise potential customers thinking of getting 900/500/300 that it'll work.... not just that it can be delivered to a hole in the metre thick granite wall my cottage is made of.
Still, there's a lot more serious stuff out there at the moment.
Cheers my friend.