i have a T1 LINE 100Mbps. My ISP create a pppoe for this running on ether10
i have a back haul antenna that is connected to my isp. if the T1 line drops i get internet from anetenna around 50Mps
but to do this i have to set the pppoe to run on another interface ether5
when the T1 line goes up i change it again manually.
you can add 2 pppoe client, one for your principal ISP and other for your backhaul link.
In the routing table add 2 routes to 0.0.0.0/0, one with distance=1 for ISP and other for your backhaul with distance=2.