Currently WAN failover will work if 1) Lower metric link goes down, or 2) Lower metric gateway is unreachable. If you see a case where this isn’t true, we’d love to know. We are actively working on improving this to also ping an internet address to detect connectivity.
Multi-WAN isn’t yet implemented. So the if WAN with metric 1 fails 2 will work but 2 won’t go to 1. When Multi-WAN rolls out it will have load balance and true failover I’m guessing.