I have an unlocked HG612 and I would like HG612 handle the PPPoE connection and bridges pure Ethernet over LAN1 so I can have a more open dedicated router handle the NAT.
What you appear to be seeking is 'Half Bridge' mode, where the modem/router handles the PPPoX negotiation, then passes the given Public IP address to its LAN port(s) (as opposed to a DMZ on a router which would provide an extra helping of NAT).
I haven't seen 'Half Bridge' mode since I retired my old Conexant ADSL router several years ago.
From memory, it was the hell of a convoluted setup to get it working, but it did work.
You have an unlocked HG612. There are loads of settings to play with. What have you tried so far?
Don't be afraid to get your hands dirty - there is no (invalid) configuration on the HG612 that cannot be recovered by using the reset button.
Don't forget to disconnect the HG612 from the telephone line whilst 'experimenting' otherwise you'll be DLM'd.