If you're out in the field and need an Internet connection, then you could connect a USB 3G or 4G modem dongle to your Pi to obtain Internet access over the cellular data network. There are millions of dongles out there, but there are two main ways they are seen by the Pi-either as a serial modem device or as a virtual network interface.
The easiest ones to work with are those that present themselves as virtual network interfaces, since they require less configuration to get going, and that's the type we will use for this project.
Note
Serial modem types, while working very well—and to an extent having greater flexibility, such as supporting the AT command set—need the addition of a point-to-point protocol (ppp) daemon and dialler software such as wvdial in order to make a connection to the Internet.
Huawei brand modems are readily available, and some older models can be bought cheaply on eBay and Amazon. Their HiLink range are the ones to look out for...