in the examples that came with the MKR GSM 1400 Board there was a sketch that make use of the "SerialGSM.read()" and "SerialGSM.write()" function (SerialGSMPassthrough). Interestingly there is no library included and the functions working strait out of the box.
So I guess the they are included in the standard library that came with the MKR 1400.
Where I can find the documentation for theses (and maybe other) dedicated functions for the MKR 1400?
Try this to start with. It's a great module to use and has plenty of examples which you can extend and change to achieve what you want with it. I'm very much enjoying playing with the MKR GSM 1400.