DFR0300-H not reading properly

I am new to this, so I might be making a dumb error here:

I am trying to read the EC value of my water. I know I have rather hard tap water, but I am not getting any values unless I go very high on the EC value.

If the sensor is dry, I get a reading of 0.075V (that is at 5V in and 0db attenuation).

If I put it in tapwater, I get the exact same 0.075V.

If I put salt in the tapwater, I get a higher reading (depending on the amount of salt, but I got 0.250V before).

If I put it in the included calibration liquid, I am getting 0.373V.

I am using a ESP32 chip (and I have to, that's a limitation, unfortunately) and I am aware, that I need to calibrate, but if the returned voltage is the exact same, no calibration in the world can fix it, right? So my question is: Am I missing something easy? Could my ESP32 be the problem? Is my sensor defective?

I appreciate any hints you could give me!

For completeness sake, here is my sensor config yaml on the ESP32:

- platform: adc

    pin: 34

    name: "EC-Sensor"

    id: water_ec

    update_interval: 10s

    unit_of_measurement: "mS/cm"

    attenuation: 0db 

    accuracy_decimals: 3

    filters:

      - median:

          window_size: 15

          send_every: 5

      - calibrate_linear:

         - 0.075 -> 0.0

         - 0.373 -> 11.40