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.