Raspberry Pi as a router using a single network interface
Amazon.com: Upgraded Duplex DMR Hotspot WLAN Ethernet Port Assembled Radio Station WiFi Digital Voice Modem Support DMR Dstar YSF P25 Fusion Antenna Raspberry Pi Model 2B Two Colors OLED 2 TimeSlots :
Add cellular connectivity to your Raspberry Pi
Tutorial - Using Cellular Modems with the Raspberry Pi 4B - SwitchDoc Labs Blog
Raspberry Pi 4G/LTE Cellular Modem Kit
SIM7600G-H 4G HAT (B) For Raspberry Pi, LTE Cat-4 4G / 3G / 2G Support, GNSS Positioning, Global Band
Raspberry Pi SixFab IoT Gateway Quickstart - Hackster.io
Mini Simplex Hat Hotspot Main Board for Raspberry Pi/BlueDV MMDVM Modem Box | eBay
Waveshare Sim7600a-h 4g Hat for Raspberry Pi LTE Cat4 GSM At&t Modem Hotspot for sale online | eBay
Cellular Modem & HAT for Raspberry Pi - 3G/4G/LTE/CAT-M1
Raspberry Pi : How to access the Internet using GSM / GPRS Modem (SIM900/SIM800)
Getting Started
Build a router with mobile connectivity using Raspberry Pi | Opensource.com