Hi Glenn - the first thing to do is clarify whether you actually need to set it up in this way. Is the setup you're trying to use like this: Modem > Netduma > Zyxel > All Devices?
It looks like your Zyxel is a Modem, not a router, so your setup should be Zyxel > Netduma > All Devices. You shouldn't need Bridge Mode with this setup. When you're using this setup and you're wired to a PC, what speeds do you receive? Do you have an open NAT on your consoles (if you play on consoles)?