Enable switching back to primary WAN after a time period

I've recently got a firewalla purple and am loving it as a device. My primary internet connection has been terrible as of late with frequent disconnects. Multi-WAN is working like a dream with a failover to my backup cellular WAN. However due to the patchy nature of the primary WAN when it goes down it'll bounce up and down for some time. Is it possible to configure the automatic switch back to the primary WAN connection only after a given amount of time? I've checked the settings and can't see anything obvious unless I put a higher requirement on the pings on the primary but assume that will delay failover from happening in the first place