The configuration tasks on DEVICE | High Availability > Settings are performed on the Active firewall and then are automatically synchronized to the Standby firewall.
To configure HA with a dynamic WAN interface
Only the switch to which the two firewalls are connected needs to be notified. All outside devices continue to route to the single shared MAC address.
Navigate to DEVICE | High Availability > Monitoring.
Select the PPPoE interface and click the Edit icon.
Interface Monitoring Settings dialog is displayed.