To control the nRF9161 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 nRF9161 Development Kit (DK) at nRF9161 DK Downloads. For more information on AT Client, see Cellular: AT Client.
Available AT commands can be found in nRF91x1 Cellular AT Commands.