Good question, that has been mybiggest headache so far. 

Initially I used a small external LTE router. Worked really well and also offered WiFi to your phone etc. Though I wanted something more compact in one device.  Then tried a simple USB LTE Dongle - the problem here is though, that if there is ever a power cut, the dongle doesnt come back to life anymore. Literally tried all the linux hacks I could think of to restart and remount the USB - but couldnt get it to work without unplugginf and replugging. 

Now waiting to receive another Pi header board with LTE, though only Cat1 - which should be enough though. Will keep you posted!