Seeed’s LTE CAT.1 Pi HAT is an open source cellular extension modem for Raspberry Pi, based on u-blox LARA-R2xx series. It compatible with Raspberry Pi 1 Model B+ and later versions.
LTE CAT.1 Pi HAT is designed for LTE Category 1 networks and with 2G fallback(EU version only). Embedding common protocols, like TCP/UDP, HTTP.
LTE CAT.1 Pi HAT support UART and USB interfaces. By UART, Raspberry Pi communicate with LTE CAT.1 Pi HAT via AT command without any special driver. It’s well suited for building prototyping rapidly.
3GPP TS 27.007 , TS 27.005  standard AT command
Other scenario need cellular networks
Wiki of LTE Cat 1.Pi HAT