The device has 7x2 connector on its PCB with the markings of the pins for SPI (CS, DO, DI, CLK) and I2C (SCL, SDA) as well as 3.8V supply and GND pins.
In the SDK, you may need to add spi-tools and i2c-tools modules, though in 7.04 firmware I found /dev/i2c-2 and /dev/spidev6.0 already present.
There also seems to be preinstalled tools for i2c interface:
i2cdetect i2cdump i2cget i2cset i2ctransfer
As is the case with SPI:
root@Teltonika-TRB140:~# spi-pipe -h
usage: spi-pipe options...
-d --device=<dev> use the given spi-dev character device.
-s --speed=<speed> Maximum SPI clock rate (in Hz).
-b --blocksize=<int> transfer block size in byte.
-n --number=<int> number of blocks to transfer (-1 = infinite).
-h --help this screen.
-v --version display the version number.
root@Teltonika-TRB140:~# spi-config -d /dev/spidev6.0 -q
/dev/spidev6.0: mode=3, lsb=0, bits=8, speed=19200000, spiready=0
I have not tested that personally, the tools were created for developers to tinker around, interested in expanding device functionalities and use cases, to try, thus hopefully the above details will provide a good starting pont.