Many thanks for this thread, as I'm trying to do pretty much exactly this (use an unlocked HG612 as a temporary ADSL modem/router), but failing!
Currently I have a very slow ADSL connection (1.5km from the exchange, no LLU). It's a new house, so I flood wired it with Cat 6 and because the walls are all covered with foil backed plasterboard wifi really doesn't work outside any room with a WAP in. When I had the line installed I blagged an NTE5 Mk3 VDSL box from the Openreach guy, knowing that sooner or later we were going to get FTTC under BDUK. So, as a temporary measure I bought a TP Link ADSL modem/router and fitted it on the wall 6 inches from the NTE5, with it plugged into a Cat 6 port with a very short CAT 6/PoE adapter (so the modem router is powered from the house main patch panel, where PoE power injectors are fitted).
The Cat 6 runs to the patch panel and thence to a 12 port Gb switch that distributes data around the house. This all works very well, although I struggle to get more than about 3Mb DL and 300kb upload, because of the length of wire to the exchange (there's actually about 3km of copper covering that 1.5km distance - rural area and the cables are far from direct!).
We're about to get FTTC, and I've confirmed that I can get it, probably at around 27 Mb DL. In preparation, I purchased an HG612, thinking I could unlock it, run it as a modem/router in ADSL mode for now, then switch it to VDSL mode when we get FTTC.
Yesterday I unlocked and fitted the HG612, set it as I thought it needed to be set to work for ADSL and as a router, and I can't get it to work. It seems to lock to the DSL signal OK, but the diagnostic message reports DSL connected but unable to get an IP from PPPoE.
I'm a raw beginner at this, really, and probably know just enough to be dangerous, but that message suggests to me that the modem has connected OK (I entered the right username and password when configuring it) but that the router part may not be working.
I'm guessing that the screenshots above (in the post by jch on September 30 2015) are for the configuration as just a modem, rather than as a modem/router.
FWIW, my current ISP is the PhoneCoop, but I'm not sure who they are buying from. The username includes "btbroadbandcomplete.com", so they may be buying from BT Wholesale. They cannot provide an FTTC service, so I'm going to have to switch to another ISP to get that in the next few months.
What I'd like to do is get the HG612 set up as a modem/router now, so the switch to FTTC is (hopefully) relatively painless, with no hardware changes needed. I'd be very grateful indeed if anyone could shed some light on what settings are needed to get the router part working and allow the existing network to internet gain access via this modem.