OK, finally figured out why #openwrt's #mwan3 was acting as a load balancer not a failover system.

The advice all over the Internet is "Give all your interfaces metrics".

This is probably true, but mwan3 will ignore the metrics unless you also do some additional set up. Specifically in MultiWan manager's Rules tabs are listed three "rules" by default. Two of these should be deleted (the IPv6 rule and the HTTPS rule. The former is silly, the latter is apparently an example, and useless.)

But the middle rule, "default_rule_v4" needs modifying to use the 'wan_wanb' policy. Edit it and that will fix the fact it's load balancing instead of failovering.

How does that use the metrics? Well, it doesn't use THE Interface metrics. Instead it uses the "Member" metrics. wan_wanb is a policy that says "Use members wan_m1_w3 and wanb_m2_w2 for routing stuff". And in Members, wan_m1_w3 is a "member" that has a metric of 1, and wanb_m2_w2 is a "member" that has a metric of 2.

It's THOSE metrics that are actually important. Despite everything I read on the Internet suggesting it was the Interface metrics that mattered.

So... I think the thing is finally working. It only took 3-4 days to figure it all out. I am not 14 years old! I am too old to do this kind of thing!

Guys, please make the documentation match the stuff you're installing.