EdgeRouter WAN

Settings Overview:

load-balance {
	group <name> {
		interface <name> {
			route {
				default
				table <name>
			}
			weight <number>
			failover-only
			route-test {
				type {
					default
					ping {
						target <host>
					}
					script <name>
				}
				initial-delay <number>
				interval <number>
				count {
					success <number>
					failure <number>
				}
			}
		}
	}
}

(Source: EdgeMAX - Dual WAN load-balance feature (v1.4.0))

minimal config:

ubnt@ER-Pro# show load-balance 
 group WLB {
     interface eth0 {
     }
     interface eth1 {
     }
 }

ubnt@ER-Pro# show firewall modify 
 modify WAN_WLB {
     rule 10 {
         action modify
         modify {
             lb-group WLB
         }
     }
 }

ubnt@ER-Pro# show interfaces ethernet eth2
 description LAN
 address 192.168.16.1/24
 firewall {
     in {
         modify WAN_WLB
     }
 }

For a more detailed look at dual WAN, but only use WAN2 for failover. Let's assume:

First create the load-balance group with eth1 as failover-only:

configure 
set load-balance group WAN_FAILOVER interface eth0
set load-balance group WAN_FAILOVER interface eth1 failover-only 
commit

ubnt@ubnt# show load-balance 
 group WAN_FAILOVER {
     interface eth0 {
     }
     interface eth1 {
         failover-only
     }
 }
[edit]

Then create the policy the the LAN interfaces will use:

set firewall modify WAN_POLICY rule 10 action modify 
set firewall modify WAN_POLICY rule 10 modify lb-group WAN_FAILOVER
commit

ubnt@ubnt# show firewall modify 
 modify WAN_POLICY {
     rule 10 {
         action modify
         modify {
             lb-group WAN_FAILOVER
         }
     }
 }
[edit]

Then apply to the LAN interface(s):

set interfaces ethernet eth2 description LAN
set interfaces ethernet eth2 firewall in modify WAN_POLICY
commit
save
exit

(Source: Re: 802.3ad failover)

set interfaces ethernet eth1 pppoe 0 user-id joe
set interfaces ethernet eth1 pppoe 0 password secret2
connect interface pppoe0
disconnect interface pppoe0

(Source: PPPoE Client - CLI Commands)

It's possible that the default route is still active. The PPPoE route can be forced on connecting with the following setting:

set interfaces ethernet eth2 pppoe 0 default-route force