The ERPoE has a hardware switch on eth2-eth3-eth4 (and a fourth port connecting to the CPU so it can route). If you look, there's a line on the front of the router connecting those three ports, to denote the switch. Delete the bridge and plug your gigabit switch into eth4, then plug the CloudKey into that switch. If your switch doesn't support PoE, then you can power the CloudKey with USB.
I seem to recall that bridging can restrict throughput to something like 10-20Mbps (though I'm sure someone will correct me with the correct number if I'm wrong).