Single WAN, two Gateway (primary and backup)

[PaoloA]

I think it is something common, but I can't find a simple and complete how-to about its implementation in a simple way.
I have a main connection uplink (50/20 Mbit) that is where usually the in/out traffic flows.
I halso have a secondary connection (4/0,5 Mbit), that I want to use as backup when the primary connection fail.
Both have public static IP that arrives in the same ethernet switch where my pfsense WAN port is connected.
I want pfsense automatically route the traffic to the secondary link when the primary fail and restore the primary when it come up again.
It seems simple, but I always find info for the multiWAN config that seem to me not good for achieve what I want.
Is there any how-to, guide, …?

Thanks, P.