Yes, you can. But you should be aware of the limitations.
First, you cannot connect a USB-modem directly into the Raspberry Pi. You need a 3G/4G/5G/LTE router. The reason for this is complicated, but in short there are too many USB-modems and mobile carrier combinations out there for us to be able to support.
Second, keep in mind that data plans can be very expensive depending on where in the world you are located. While we have put a great amount of effort into minimizing the data consumption by Screenly (such as extensive caching), the nature of HD video and image assets is that they are large files. As a result, you should keep this in mind when you sign up for your mobile data plan.
There are a large number of mobile routers available, and the brands/models available varies by country. Generally speaking, as long as the router can provide you with an internet connection over either Ethernet (preferred) or WiFi, you should be able to use Screenly.
Here are some options that all allows you to connect your Raspberry Pi to the router using Ethernet:
You may also look into IoT specific carriers, such as Soracom or Twilio.
Please note that we do not officially endorse any of the products listed above.