How to include a .dll using Qt Creator - qt

Question

I've followed the steps described in https://wiki.qt.io/How_to_link_to_a_dll
but, somehow I still get undefined reference errors.
This is what I did:
add to my .pro file the library folder path via INCLUDEPATH+=
add to my .pro file the .dll via LIBS+=
include the .dll (in my case "okFrontPanelDLL.h") via #include in my code
I don't know if it matters, but my library is taken from: http://intantech.com/files/RhythmStim_API_Release_170328.zip
and the extracted folder contains a single .dll and multiple source and header files (do i have to add all sources and headers from the library via SOURCES+= and HEADERS+=?).
Currently, I can declare a variable based on a class defined in the library
okCFrontPanel *dev;
but accessing functions defined for the class, e.g. calling the constructor like
dev = new okCFrontPanel;
leads to an undefined reference error.
edit: I tried direcly adding the source and header files form the library into my Sources folder instead of linking the library and the code works fine, so there is (probably) at least nothing wrong with how I am trying to use the functionalities of the library.
edit2: further information:
OS: Win 7 64 bit
Qt version: 5.9.0
compiler: MinGW 32bit
file location: /[PROJECT FOLDER]/mylibrary
#include <QCoreApplication>
#include "myLibrary/okFrontPanelDLL.h"
int main(int argc, char *argv[])
{
// QCoreApplication a(argc, argv);
// return a.exec();
printf("hello world\n");
okCFrontPanel *dev;
dev = new okCFrontPanel;
// dev->BoardModel;
// If only one Opal Kelly board is plugged in to the host computer, we can use this.
dev->OpenBySerial();
// Set XEM6010 PLL to default configuration to produce 100 MHz FPGA clock.
dev->LoadDefaultPLLConfiguration();
// Upload RhythmStim bitfile which is compiled from RhythmStim Verilog code.
dev->ConfigureFPGA("mylibrary/main.bit");
printf("omg dis is working\n");
}