I have never worked with a Arduino MKR GSM 1400
anyway for solving the problem you should reduce the potential sources of bugs as much as possible.
In your case this means taking any other device that you know for sure that the communication over a serial interface works reliably.
Test the sensor with this device.
This could be an Arduino Uno or a USB-to-serial adapterboard.
You should test the Arduino MKR GSM 1400 itself the same way.
Connecting the Arduino MKR GSM 1400 on your second serial interface to another device that is well known for send/receive serial reliably.
Another option is to wire a loop-back which means connecting Tx with Rx of your second interface directly through a wire and then upload a testprogram that sends something on the second serial which should then immetiately received over the Rx-pin.
This code should echo a lot to the serial monitor so you can see what your code is doing.
Edit: another way is to connect such a 24 MHz 8 channel logic analyser
it works with a freeware called sigrok pulseview
best regards Stefan