Using Zyxel SCR 50AXE as the secondary router for Bridged Connection

We're trying to set up the Zyxel SCR 50AXE as a secondary router for a connection currently using a BT Smart Hub 2. We would like the BT router to act as the Bridge.

When the BT router is in bridge mode, no configuration options we try on the Zyxel seem to give it an internet connection.

We have tried DHCP, Static with the Gateway looking at the BT router, PPPoE for the actual BT connection.

The only reason we need the BT Router is because the Zyxel doesn't have ADSL Connection. It's very hard troubleshooting as the router has to be connected to the Nebula Cloud to make any changes and then these changes take a few minutes each to propegate.

Any advice?