Sorry, I don't know off-hand how to do that using modern hardware and software. I've done it in the past on the hardware from 2003/2004.

You should seriously consider just buying a better gateway. Either buy Asus DSL-AC68U to replace your BT jobbie, or:

3) reconfigure your BT jobbie to a bridge and and a plain Ethernet gateway like Asus RT-AC68U.

4) hire somebody locally to consult you.