@ZeeBoussaid it was in the closed beta for MX16 but isn’t there in MX16.4 that went to public beta. It’s still a feature that is ‘coming’, hopefully within the next few months (we’ll see). It probably wasn’t as stable as Meraki wanted in the closed beta.
So yes, if all things align, you will be able to use the cellular modem in the MX67C as a full WAN link, not just as failover. But remember it will be 1x Ethernet WAN and 1x LTE WAN (there won’t be an option for two Ethernet plus LTE all as WAN ports).