You have a number of options when connecting over cellular (LTE) with your LiveU Solo:

USB Modems

These devices connect to the Solo over USB, and connect directly to various LTE networks.  Carriers around the world and in the US often sell them directly, with plans or to attach to your existing plan.

In countries where its still common for cell carriers to sell SIM cards direct, you might be able to buy them at other stores as well and later add a SIM.

In the US, you can not buy just a SIM with just a few exceptions, so you have to buy the device direct from the carrier. They are also not commonly carrier at the local reseller stores, since those have limited stock and supply.  Its easiest to order them directly online.  See the related article about where to find modems to order line in the USA.

MIFI

You can connect to a MiFi using the Solo's WiFi capability.  Just connect as you would any other device, such as your phone. This is a great way to add a third connection or to use a single connection, since MiFi's are very common.  Note, you will need to monitor and charge the battery of the MiFi independently of the Solo, so it does add a bit of work to your production!

Cell Phones In Hotspot Mode

Basically just like the MiFi above, but again, another good option to add a connection to get to a bond of 2 or three carriers, or in a pinch when its the only connection you have.

Routers With Ethernet Connections

One last, somewhat new and more exotic option is the Netgear Nighthawk LTE router which has both an internal battery and an ethernet port - so you can connect the solo over ethernet and use the Nighthawk's LTE connection.  Again - this is a great way to add a third LTE connection, or in any combination.

To summarize:

• You can use many different style of connections with your Solo to get it onto LTE networks
• Talk to your country's carriers to find the devices you need
• Try your carrier directly (via their website) rather then local stores, as local stores tend to focus on mostly phones

A partial list of supported modems can be found here.