Fail-over default route

Good morning all,

Got some good advice on here last week and hoping to double up.

I have a main site with several remote sites running off of it. Currently, I have an NLAN with my ISP providing internet to the remote sites via the main site with 20Mbps speeds. We’re implementing a Microwave network to provide much faster speeds and improve reliability as the John Deere cable locator is a frequent problem for us. Both networks would operate EIGRP. I need to have both up and running at the same time so if there is ever a failure on the MW network, it could failover to the NLAN.

My question is: for the remote sites what do I put as the default gateway on their routers? Currently, their router of last resort is the main site’s NLAN interface. However, if I change that to the main site’s MW network interface and that network ever failed, how would it know to switch over to the NLAN’s network? Obviously, I only want the traffic to go over the NLAN if the MW network is completely down so not really a load balancing issue.

I’m thinking maybe make a loopback address on the main site’s router and advertise that on both EIGRP networks and then put that as the default gateway for the remote site’s routers?

Any guidance would be appreciated!