This is TPB23 driver of SERCOM. You can use this dirver for NB-IoT of LGU plus(LGU+) in Korea.
The NB-IoT module's seller provided a document on how to start based on Mbed OS 5.11, created by CodeZoo). This driver support latest Mbed OS(5.15.1), and you can find example code from mbed-os-example-cellular-TPB23.