If you turn FTTP on in the hub's management pages and if you are still on a working VDSL line and you have the hub plugged into your phone line nothing will change. The VDSL broadband will still work and you can happily turn it on or off without affecting the VDSL broadband line.
If you have been changed to FTTP and you have connected port 4 of the hub to the ONT and the ONT lights are all green and you have checked that FTTP in the management pages is turned on your FTTP connection should start.
If it does not make sure you have unplugged your phone line from the hub.
If it still does not work you would appear to have a problem that will require you to report a fault to BT.
I give up, you are not listening to the answers to your questions.
You should already have plugged into the fibre socket otherwise how do you expect your new fibre to work!
I have already explained that if you turn on the Fibre in thd hubs settings IT WILL NOT affect your "old" broadband or its settings.