Setting up Untangle outside network

What we are trying to accomplish is setting up an Untangle machine as a transparent bridge on the outside of our network.

We are currently experiencing a DDOS attack on our internet line.
The current router we have, a Zyxel Zywall 200, can't handle the connections.

What like to do is setup the Untangle machine in front of our Zyxel so it can pass through anything but block the DDOS attack.

The Zyxel is handling NAT en multiple VPN's so it has to have our WAN ip-address.

Is it possible to set up the Untangle machine as a transparent bridge but have our WAN ip-address on the Zyxel?
We only have one ip-address available so it is not possible to assign a different public ip-address to the Untangle machine.

This is the setup we would like to have:

Modem <--> Untangle <--> Zyxel <--> LAN

I hope you can help us.

With kind regards,
Luc Verhoeven