Hello,
I'd like to introduce you a fairly new script that manages failover WAN connection - WanMonitor.
I have written it because, mwan3 monitors all links and thus eats mobile data and simplefailover does not support PPPoE connections. This script addresses both problems, allowing to set PPPoE connection as primary WAN and LTE as secondary failover connection.
More details and configuration sample can be found on github: https://github.com/belliash/wanmonitor