Hi,
This seems to be challenging/interesting.
If you are referring to plugging a device (3G/4G supporting) into USB port of MX.
I believe only supported devices and configuration are listed in the following url.
https://documentation.meraki.com/MX-Z/Cellular/3G_-_4G_Cellular_Failover
You may use a third party device (may be a router) which can convert 3G/4G to LAN and plug the same to into the WAN Port of MX (Incase you are looking forward to use 1 of your WAN Port)
USB tethering I believe may not be possible with MXes.
Regards,
Ajit
AjitsNW@gmail.com
www.ajit.network