Hi @guitb,
I agree with @jimmyt234, please open a case with Support to request your network(s)/org is added a checkbox on the Firewall page to enable/disable cellular failover rules. You may reference this KB in the case description so we're aware of exactly what you're looking for!
The feature allows the Cellular Failover Rules to be used for when the non-primary uplink is activated. For example, if WAN 1 is primary and fails over to WAN 2 and the MX has this feature applied, the cellular failover rules will apply to traffic going over WAN 2 instead of the default Outbound rules. This is useful for customers that have a cellular to Ethernet bridge or a metered connection on their non-primary uplink.
Cheers,
If you found this post helpful, please give it kudos. If my answer solved your problem, click "accept as solution" so that others can benefit from it.