To control the nRF9160 LTE modem with AT commands, an application program that routes AT commands from the computer is needed. Nordic offers the AT Client application for this purpose. The AT Client firmware needs to be compiled according to the product hardware pin mapping.
nRF Connect SDK includes AT Client's source code. The compiled AT Client firmware is available for the nRF9160 Development Kit (DK) at nRF9160 DK Downloads. For more information on AT Client, see nRF9160: AT Client.
Available AT commands can be found in nRF91 AT Commands Reference Guide.