How to setup a USB modem with RPI3 (Of course, with OpenWrt) - Network and Wireless Configuration - OpenWrt Forum
Amazon.com: Dilwe 4G LTE USB WiFi Modem, LTE/4G 150 Mbps USB Mobile Module with SIM Card Slot, High Speed Portable Travel Hotspot Mini Router Supports USB, UART for PC, Industrial Computer :
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VoCore2: Support USB 4G LTE | Vonger's Blog
4G/5G/cellular WAN on OpenWrt - Traverse Ten64 Documentation