To build on @ww answer; if this was an MX67C you would need to plug it into the Internet via its WAN port first to allow it to upgrade its firmware. After that you can run on cellular only.
In this case you say an MX67, so I'm going to assume you are either using a USB 4G modem (in which case it will just go as long as the firmware already supports your USB stick) or a 4G router (in which caser it will always work).
But it wouldn't hurt in either case allow the MX to upgrade itself over a fixed line broadband connection first before deploying it.
Hi Philip, thanks for the detail. Yes we are using straight MX67s with a USB 4G modem. We have a few sites were we have a kiosk in a shopping center where we can only use 4G for the network connection, or where we can't get an IPVPN or it's too expensive and we want to use the 4G as a backup connection if the Internet link fails. Can you build AutoVPN tunnels in both of these scenarios, either as a primary 4G connection or failover AutoVPN traffic to cellular?