LB2120 between a modem and a router

I have a modem/router that passthrough the pppoe connection to my router (edgerouter).

In this case one internet connection is initialized and maintained by the router, and another one by the modem router (D-Link DVA modem router) and used mainly for a voip connection.

All is working fine but I would like to add a failover line to the router connection.

I don't know if the LB2120 can work in the between of a pppoe connection (modem <--> router).