ip route Failover Priority with Two Backup WANs

[emuman100]

I have three WANs; one is the primary and the other two are backups. 

Primary = dhcp; Secondary = Cellular0/2/0; Tertiary = Dialer1

The routes are as follows:

ip route 0.0.0.0 0.0.0.0 dhcp

ip route 0.0.0.0 0.0.0.0 Cellular 0/2/0 251

ip route 0.0.0.0 0.0.0.0 Dialer1 254

Cellular 0/2/0 interfaces to the cellular provider's CG NAT. When the primary goes down and traffic cannot traverse through Cellular 0/2/0 while the interface is still up, I wanted it to go through Dialer1, but it goes through the cellular interface instead.

All of these use route_map for NAT and NAT works as it should for these interfaces. The issue is route priority. Cellular0/2/0 is the secondary WAN. When it's called up due to traffic, it gets an IP address via CG NAT, but won't always pass traffic when the cellular account does not have prepaid funds, but, the router sees it as an interface that is up and has an automatically assigned a CG NAT ip address, so my route priority defined by administrative distance goes to Cellular 0/2/0.

When traffic does not pass due to no funds in the cellular account, I had expected the Dialer1 254 route to be used, but that isn't the case. 

When the primary WAN goes down, the interface actually goes down, so IOS-XE removes that route from the routing table, so the next closest route is Cellular 251. When traffic cannot traverse Cellular 0/2/0 due to lack of funds, the interface still stays up and still has an ip address, so the route remains in the routing table. If it were to go down, the route would be removed and traffic would traverse through Dialer1, but in this case, Cellular 0/2/0 still stays up and gets a CG NAT ip address. 

What options do I have? Can I use an ip sla and a track somehow or is there something else I can do which would work more reliably?

Thanks.

[emuman100]

I wanted to post an update. Using an ip sla and a track of the route with appropriate route_map for NAT, the failover was seamless, but this is only partial testing. I did not test the ip sla when Cellular 0/2/0 can pass traffic, but when it can't, the track removes the route and administrative distance of the next route of the dialer interface routes traffic. 

The trade off here is that the ip sla always generates traffic, so it ticks away at the bandwidth bucket of the cellular account. But, I don't think there is any other option here, is there? The WAN links don't utilize any routing protocols like BGP, so using some ip sla seems like the only way I can do this. But for down detection, what sort of options are better than icmp ping?

Thanks.