We have just purchased a Draytek 2860Ln for a customer and have a question regarding Multi-Wan Failover. The router will be setup as follows:

WAN1 (VDSL)
WAN2 (Ethernet connected to VDSL BT Modem)
WAN3 (LTE/4G) - Set as failover if WAN1/WAN2 down

I’m going to setup a router policy so that traffic from LAN1 Subnet (Data/Computers) goes out on WAN1 and LAN2 Subnet (VOIP/Phones) goes out on WAN2. What I want to happen is that if WAN1 goes down the traffic fails over to WAN2 and vice versa but if both WAN1 & WAN2 go down then both LAN1 and LAN2 traffic will go out on WAN3.

In the route policy I can set a failover rule to a WAN other connection but how do I then set another rule to the 4G if that connection is down?

Both WAN's need to be set to always on, and the data SIM as a failover.
Set the rule, if WAN1 and WAN2 are down, to enable the Data SIM.
Set Pings on each WAN to say Google, so that the router will failover quickly.

Under Route policy, set the Failover WAN as "Default Wan", this will allow the failover to the Data Sim, should both WAN's go offline.