unlink balancing with bridge

We just added a 2nd ISP and want to balance traffic between it and the original ISP.

ISP1 is connected in bridge mode to our perimeter network (boxes on the perimeter network are assigned IP's from the netblock provided by ISP1).

I connected ISP2 to another interface on the Astaro box and enabled uplink balancing between the existing ISP1 bridge interface and ISP2's interface.

All traffic from the perimeter network still goes out via ISP1 regardless of balancing weights or multipath rules. How can I balance traffic between the bridge and ISP2 interfaces?

Thanks,
Alan