Set up a router in half-bridge mode for PPPoA connection

I need to set up a router to half bridge mode for PPPoA connection in my office in central London. At the moment I have a single-port Zyxel P-660R-D1. I spent days trying to set it up in such a way but I can't manage to. Zyxel support, ISP support and various forums keep on giving me contradicting ways of setting it up, I tried them all but I still can't manage. Buying another router would not be a problem.