I’m afraid I don’t know the answer to that.
There was a discussion in the old forum on wan failover. Maybe there is something there that can help you if you haven’t already found it. Beyond that I don’t have any other info.
https://forum.ipfire.org/viewtopic.php?t=5033
I will look at converting this forum posting into a pdf and add it as a link to the Internet Connection page as the old forum will eventually close.