This document describe my idea how to configure routing on one Cisco router with two WAN lines from different ISP.
The diagram below presents this situation. Let's say that WAN from ISP1 - 18.104.22.168/30 has bigger bandwidth and we want to use it as primary line. WAN from ISP2 - 192.168.2.0/30 is a backup line.
ip sla 1 // we are checking availability of DNS server from TPSA (22.214.171.124) with source of WAN ISP1
icmp-echo 126.96.36.199 source fa0/0/1
frequency 3 // we set frequence of probes for 3 seconds
ip sla schedule 1 start-time now life forever // lets start it
track 1 rtr 1 reachability // starting tracking
ip route 0.0.0.0 0.0.0.0 192.168.1.1 track 1 // if track 1 that checks availability of DNS server is true, the default route is set to use WAN ISP1. Whenever it fails, the static route will be disabled and backup default route with AD 200 will be used.