This dirt cheap 4G LTE modem on a USB stick can be hacked to run mainline Linux - Liliputing
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How to Connect a USB Modem to a Wireless Router
USB Modem : Functions, Features and Types
Inseego USB 8 | Inseego
External USB 2.0 Fax/Data Modem 56K V.92 - Bluetooth & Telecom Adapters | Networking IO Products | StarTech.com
3. Installing 4G USB modem to Raspberry Pi – RC Bellergy's Drones