FirstRate FST600-400A 2 Wire Analog Sensor
FirstRate FST600-400A comes in 3 variants. Analog Current, Analog Voltage and ModBus Serial. All the variants have different wiring and working.
Sensor with current out with 4-20mA only has 2 leads from the sensor cable. Vcc and Iout. To complete the circuit GND is needed. Most of the Industrial Controller/PLC's need Output and Ground or close connection.

Above picture illustrates wiring structure from 2 different vendors.

Make connections based on the above picture.
In Omron NX-AD3204 range of adc for 0mA to 20mA is 0 to 8000. Sensor Temperature range changes from 0°C to 300°C to 0°C to 1200°C. Most common one is300°C. For 300°C Analog value would be 8000 and for 0°C would be 0