The current new built-in cellular models only support Verizon or AT&T. You mentioned you have Sprint, so these won't work.
There are a variety of USB based cellular cards you can get, see the list here:
Scroll down until you see 'Supported USB Modems':
https://documentation.meraki.com/MX/Cellular/3G_-_4G_Cellular_FailoverI however would suggest you get an LTE hotspot from Sprint, that has an Ethernet connection. Not sure if they have these models, but I know Verizon and AT&T both have them.
If you get one of their hotspots that has an Ethernet port, then you can avoid any hassle/bottle neck issues with the USB's. You would simply plug it into the WAN port on any model MX you choose that fits your needs. Like an MX64 or MX65 which would be less expensive.