MAX HD2 IP Passthrough on wired WAN

HI
I’ve successfully used IP Passthrough on a cellular connection, but can’t make it work on a wired WAN. We have a FIOS connection with dynamic IP and I understand that I can’t use drop in mode without a static IP.
I’m running 8.0 firmware and I see the option is there for IP Passthrough on the WAN ports but can’t make it work.
Any suggestions gratefully received.
Many thanks
David