Perhaps instead of me just telling you have a go for yourself, the Peplink GUI is pretty straightforward and all of the options you need are there and that way you will learn how the product is working and what options are available for you to use.
These articles are probably worth reading:
Personally I would leave it to always on, and then use outbound policies to control how traffic uses the connections, this assumes that there is not some great difference in the performance or cost of using those WANs though - for instance if one was linked via cellular or satellite with a high price for data transfer I would maybe not - you need to make your own determination here about what is the most appropriate setting for your environment.