Yesterday I was tinkering with some components I bought a while ago and came across the Seeed Solar charger shield (or something like that). The shield connects to a solar panel and a LiPo battery - charging the battery with solar power and in turn powering the device. I got it connected to the MKR GSM to provide power to the device and it works perfectly! Now the project is more sustainable and can survive for as long as needed in the field without the need to recharge.