If you are using a USB cellular or the embedded cellular modem in the MX i don't think you can. However, if you have a Cradlepoint or other 4G/LTE router plugged into WAN2, you can do that. We learned from our SE that you can have Meraki Support apply the cellular failover rules to WAN2 so you can control what goes out over the cellular device. For sites where we do this we have WAN1 as the primary uplink, load balancing turned off, and have the Active-Active VPN set to disabled to save on cellular use. We take a brief hit while the new tunnels come up, but in our use case that's OK. If you can't take the hit you may want to enable Active-Active VPN, but you will consume more cellular data (use at your own risk).
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