After evaluating options I have, I came up with a solution as below. Since ER605 doesn't send any traffic to backup WAN port unless there is a failure in Primary WANs. So I have marked WAN/LAN2 as...
@Somnus Thank you very much for your suggestion. Unfortunately, I don't have any control in the firewall in the Cooperate side. So marking WAN/LAN 2 as a LAN port and do what you have suggest is not...