Cannot set mode to DHCP or PPPoE when HA is on.
Note: this is allegedly supported in 5.2.1 or higher, but there are other problems with this firmware that prevent me from confirming this.
Update: With 5.2.2 I can at least set the option, I don't know if it actually works yet.
Update 2: I can do PPPoE with a HA cluster. Fail-over has not been tested as of yet.