Configuring PPPoE interface on a SRX550 Cluster

We have a SRX550 Cluster in place, which connects to a EX4200 Stack. From the EX4200 we have anuplink to a cable modem in brdige mode. This works fine, no issues. However, we have a back up PPPoE/Broadband connection. Therefore I need to create a PPPoE interface on the firewall.

I've been given conflicting information on this. As the uplink to the PPPoE router is coming from the EX4200 Switch, where do I create the dialing interface? from what I can see I can't set a reth interface as the underlying interface for the PPPoE connection.

Any pointers are greatly appreciated.

Thanks