For this example I will have 2 devices device A that is connected to the ISP and device B that is going to have the fail over route installed. for this instance lets say the address on the wan/ISP side is 1.1.1.1 and the ip address on router A going from router A to be is 192.168.10.125
What you will need to do on router B is set up a 0.0.0.0/0 route with 1.1.1.1 as the gateway address.
To set up the first route go to IP>route open up your current route and click the copy button, from there you can edit the copy.
Initially this route will be unreachable due to the router not knowing how to get to 1.1.1.1 but we can fix that by adding a second route.
The second route you will need to have will have the destination of 1.1.1.1 and the gateway of 192.168.10.125, this should come up as routable once applied but the initial route will still unreachable.
The first route is unreachable because we have not told it it can use other routes to look up the gateway address, they way it does this is through the scope and target scope metrics. The target scope tells the route that if any other route has a scope that is less than the this it will be able to use it for a lookup.
So to get this working we will either need to change the target scope on the first route to be more than the scope value on the second or you will need to change the scope on the second route to be less than the target scope on the first.
After you have done this you will need to go to your DHCP/lte etc client and make sure it is set not to add a default route any more
Once this is done you can make sure that you have enabled check gateway=ping on the first route and this should now mark the route as down once the 1.1.1.1 address is not reachable :)
You can see that the route has been set up correctly as it will be reachable and on the gateway it should say it is recursive