If you just want a single computer connected there are two ways.
One is using the setup as in the post above; with the HG612 making the PPPoE connection. I am afraid I'm not quite sure why that setup isn't working for you.
Alternatively, use the HG612 in its original configuration (locked or unlocked) as a pure modem, and make the PPPoE connection from the computer connected to the HG612.
On Windows that means that you must go into Network Settings and create a new network. Configure it for PPPoE, with a username of 'email@example.com' (actually there are lots of others that will work), and any password you like.
BT Infinity 2, moving to PlusNet (26th Sept if all goes well)