The official response from the documentation:
https://documentation.meraki.com/MX/MX_Overviews_and_Specifications/MX67_and_MX68_Overview_and_Speci...
Can I change the antennas to improve my performance?
The MX67C, only Meraki antennas are supported. Replacement antennas will be available for purchase.
Meraki only have the one antenna at present and there is no official guidance. That being said, using accessories to extend the antenna will not void the warranty, but support will just be unable to assist with any issues relating to the antennas specifically. In theory any LTE antenna with a RP-SMA should be fine.