Check to ensure the external interface on the Master and Slave node have been configured on the HA configuration page. If the External interface is greyed out on the Slave node, uncheck the box for the external HA interface, and then immediately check that same box, the field for the the external interface should now be able to be edited with the external interface for the heartbeat on the slave.
Once done, synchronize the configuration of the Slave node by clicking the "Synchronize Configuration" button on the top of the page. A window may appear confirming whether or not to continue, click "okay".
Wait approximately 5 minutes and then reboot the Slave node via the HA page. Once the slave is rebooted the HA Redundant Heartbeat status should be successful.