This is my post in another forum about the very same topic.
In a few word: I'd like to use a USB 3G dongle as a backup of my main PPPOE xDSL conenction.
So far I've been able to bring both up and running with a shent of load balancing and a few issues.
Unless I've done something wrong (which is possible as the WiKi pages about this are confusing to me at the language level), the recovery procedure seems not to work.
And, yet, the fail-over configuration is far from my understanding.

Is there anyone trying to do the same? Is he/she willing to share the experience with me?

TIA.