First remove any existing Gateway of last resort
no ip route 0.0.0.0 0.0.0.0
Add gateway of last resort (Primary)
ip route 0.0.0.0 0.0.0.0 1.1.1.1 1 track 1
Add secondary gateway (backup line)
ip route 0.0.0.0 0.0.0.0 2.2.2.2 2 name DSL_Backup
//Setup unrechability detection for redundancy
//Setup monitoring of Primary line with ping to 8.8.8.8
ip sla 100
icmp-echo 8.8.8.8 source-interface Gix/x
frequency 10
Start monitoring Primary line now
ip sla schedule 100 life forever start-time now
Finally add a track entry for sla to cause the gateway switch if Primary fails
track 1 rtr 100 reachability
Check your configuration by executing below command
show ip sla statistics 100
Check the current Gateway of last resort
show ip route
Disrupt the Primary line and see if your configuration works. Your secondary line will kick in after 10 seconds. Primary gateway will always preempt so when your Primary line comes online, your Gateway also changes to Primary gateway.