Are you on ADSL2+ now, and hoping to move to FTTC/VDSL2?
The settings to change are available in the HG612 GUI, but I don't remember them all. The first one is that currently it is probably configured for VDSL2, which is PPPoE. If you are on ADSL2+ it needs to be PPPoA. Then the VPI/VCI setting needs to be correct.
I think that's all. Google or your ISP's connection instructions or several previous posters here should be able to tell you the VPI/VCI, and whether there is anything else to change.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. Sync 72213/12265Kbps @ 600m. BQMs -
IPv4 &
IPv6