I have a Dasan router from Airtel fiber, airtel has setup its port 4 for pppoe passthrough/bridge mode. As far as i have tested it's NAT isn't functioning now and pppoe login is done on secondary router but I am not sure if it's firewall is still active or not or whether it's processing any pppoe packets or simplely relaying i can access its 192.168.1.1 management portal though.