The official answer is:
Can I change the antennas to improve my performance?
The MX68CW has fixed antennas for Wi-Fi and LTE that cannot be swapped. For the MX67C, only Meraki antennas are supported. Replacement antennas will be available for purchase.
If my antennae are lost or damaged, can I use 3rd party antennae that fit on the Meraki device?
Only the Meraki antennae are supported. Lost or malfunctioning antennae can be replaced by contacting Meraki support.
That said, the connector on the MX67C is RP-SMA so you're welcome to try. People have done it. Keep the cable as short as possible though. See these topics: