Sprint currently seems to support this usb modem Franklin U772 USB Modem
However is seems that the Z1/Z3 or MX doesn't who do I need to speak to get this support added.
Our customers would like the sprint service as the price point is acceptable and modem cost is very low.
You best path to requesting integration of a new USB modem is to contact support and make the request with them, as well you could use the "make a wish" link on the dashboard to submit your request. You could also work with your sales rep, if your project is of considerable size you may sway someone.
Honestly, getting a modem integrated with Meraki is a long process and takes a considerable amount of effort. I would suggest you explore the currently integrated options which are listed here... https://documentation.meraki.com/MX-Z/Cellular/3G_-_4G_Cellular_Failover or use a Cellular modem with a copper Ethernet hand-off.
Don't bother trying to use USB 3G/4G modems. They are not worth the hassle.
Use a unit that has an Ethernet port on it, so you can just plug it into a WAN port on an MX.
Unfortunately, you're going to want to look into a different device that's already been approved/certified. The time it would take to certify a new device is likely going to be longer than you're implementation timeline. We have 900+ MX's in the wild using primarily U620L's & UML290's with Verizon service. There are issues with the connection status in the dashboard reporting 'Ready' when in fact there is not a full cellular signal. Our ticket made it's way into the Meraki 'blackhole', however there has been some progress recently on fixing this.
I certainly respect PhilipDAth's point of view; However, I also understand that depending on the level of need (or SLA) for failover, it just doesn't always make sense in terms of cost and ROI. Our modems were almost entirely refurbished for around $40 a pop. We've had sites running multiple weeks on these while we work with the local carrier to resolve their issues. Reliability is definitely a case-by-case basis though. Certain USB modems can be outfitted with external antennae if the reception is just too poor.
I'm not going to offer up any insider information (because I really don't have it), but there have been rumors about Meraki releasing a security appliance with built-in cellular abilities in the near-ish future. If that is in fact true, I have a feeling that's where the focus/engineering is being placed at this time.
When you say the three big ones @Cowboyflr - none of them even operate in my country. Globally I suspect they may not even make the top ten list.