I would need to login to the device again and see the relevant settings page, but I strongly advise against this.
The Huawei HG612 was locked down in modem only mode for a reason. It just doesn't have the grunt as a router.
When using the HG612 as a modem and only handling the PPP (DHCP, QoS, Firewall all off) I was never able to reach above 60Mb, ever. During heavy usage that would drop frequently to 20Mb.
Your router has a dual core 1.7Ghz CPU.
Why on Earth would you want the HG612 to handle the PPP?
edit: incase you wish to try it
On the left, go to Basic > WAN
Under Port Binding untick LAN 1 and LAN 2
Connection Mode: Route
Connection Type: PPPoE
User name: Your FTTC username
Password: Your FTTC password
from old notes