One thing I have noticed is that you don't have a header in your libraries with same name as library. This makes library resolution for arduino much easier and more accurate.

For example ArduinoFixedString lib should ideally have ArduinoFixedString.h, this would avoid a FixedString.h being found in some other library. This is not the issue in this case but it causes other confusions.

For visual micro shared projects there is also currently a bug that is resolved by adding src\SimcomGsmLib.h to the SimcomGsmLib.h library. Then in your .ino code #include <SimcomGsmLib.h>. Aside from the vm bug it is good practise anyway, especially for users who install many different libraries into documents\arduino\libraries.

You have not posted an example project or said which board or toolchain you are using for your test. That would help thanks along with link to the library that can not be found.