Muddling through myself, documentation is a bit lacking. I think the bit you need is that (from the bottom of routing page, groups tag)```
Note: Remember to use these Gateway Groups in firewall rules in order to enable load balancing, failover, or policy-based routing.
Without rules directing traffic into the Gateway Groups, they will not be used.
Going from the default setup, I added a rule to LAN below the "anti-lockout" and above the "default allow LAN to any" Which is almost exactly the same as the "default allow LAN to any" except the "advanced" section at the bottom has the "Gateway" pointed at the Gateway group.
I have some other things going on right now with my setup that are not yet working, but I think that part is basically correct…or at least worth a shot.
I think (unless you really don't want to use one connection until the other fails) that you might really want both at the same tier, but with different weights (proportional to their speeds) defined (which is back on the gateway, not the groups.) i.e. if one is 1MB and the other is 50MB, one gets weight 1 and the other gets weight 50.
Better information welcomed.