I do it with a lan cable from lan2 to my switch, bypassing the smoothie. There is no security issue as lan2 does not have any access to the WAN, the cable is purely to access the hg612 UI and telnet for stats purposes.
I had a look at this today on LAN2 on the modem I can get a public IP address and get on the internet with my set up .
So I do not think this will work on ADSL modem !.
I did try again to set up PPPoE, Bridge, Smoothbox dialing, I could get DSL up but there was no way to get it on the internet, I also tried my laptop PPPoE but that would not connect it.