In order to use the multiwan feature, you need to do a number of configurations.
The order of operations involved in configuring MultiWan is roughly the same as the order in which the interface displays the setting tabs.
As a first step, you need to add all network interfaces that should be part of the MultiWAN.
Next, each interface must have at least one member, with per interface giving it appropriate Metric and Weight.
With the members set up, you must create at least one policy containing at least two members.