I've been trying the Huawei E3272 and E3276 USB dongles..

Anyway, some other hardware ideas I've come across and will investigate:

PiAnyWhere 4G & LTE Hat for the Raspberry https://www.pianywhere.com/product/pianywhere-4g-raspberry-pi-hat/ £160

PiAnywhere 3G https://www.pianywhere.com/product/pianywhere-3g-raspberry-pi-hat/ £100

PiAnywhere GSM https://www.pianywhere.com/product/pianywhere-gsm-raspberry-pi-hat/ £60

4G + GPS Shield for Raspberry Pi – LE910 - (4G / 3G / GPRS / GSM / GPS / LTE / WCDMA / HSPA+) https://www.cooking-hacks.com/documentation/tutorials/4g-gps-lte-wcdma-hspa-3g-gprs-shield-arduino-raspberry-pi-waspmote-tutorial/ E444 (euros) <- this was is pretty great because of the amount of software they've already written so it's super-easy to use! But very expensive!

Note that I'm only using 4G or 3G for debugging with prototypes, the final elephant detectors don't need to upload their images. They only need GSM to communicate with deterrence devices and local people using SMS.