MLPP with Peplink 380

Sorry folks, maybe my previous question sounded a bit too complicated. In essence I need to know if any of you have successfully used peplink with MLPPP routers using OpenWRT firmware. If so, have you had double-NAT problem - and what did you do to overcome it? The idea is to use Peplink as the firewall and not the mlppp router (i.e. no drop in mode please)