Missing functionalities when deploying Qt application

I have created a deploy folder for my Qt gui application, I have added all DLLs it screamed for. The app is running, but there 2 functionalities that are missing.

void MainWindow::on_action_About_triggered()
{
    QString filePatch = QApplication::applicationDirPath() + "/changelog.txt";
    QFile f(filePatch);
    if (!f.open(QFile::ReadOnly | QFile::Text))
        return;

    QTextStream in(&f);
    QMessageBox::about(this, tr("About testapp"),
                getAppVersion() + "\ntestapp\n\n" + in.readAll());
}

And

QPrinter printer;
    printer.setFullPage(true);
    printer.setPaperSize(QPrinter::A4);
    printer.setOrientation(QPrinter::Landscape);

    if (SpecialTypes::printType_t::ePrint == pType)
    {
        printer.setOutputFormat(QPrinter::NativeFormat);

        QPrintDialog printDial(&printer, this);
        if (printDial.exec() == QDialog::Accepted)
        {
            textEdit->document()->print(&printer);
        }
    }

Both dialogs are not showing on a computer with the deploy folder. When I run this in Qt creator on the pc I am building the app on, those dialogs work properly. I guess I need to include some additional libraries, but I have no idea which ones, as the app doesn't throw any error, it just doesn't show the dialogs.

Your problems have nothing to do with libraries.

The first method, obviously, returns here:

if (!f.open(QFile::ReadOnly | QFile::Text))
    return;

The second one doesn't get inside of

if (SpecialTypes::printType_t::ePrint == pType)

With the first one I'd recommend you to print to log the file name, and, if this is the case, change code to this:

QDir dir(QApplication::applicationDirPath());
QFile f(dir.absoluteFilePath("changelog.txt"));

If the problem isn't connected to the file path, then you should check file's permissions. And write something like this:

if (!f.open(QFile::ReadOnly | QFile::Text)) {
    qDebug() << "Error opening file. Error code =" << f.error();
    return;
}

For the second one you should definitely add:

} else {
    qDebug() << "SpecialTypes::printType_t::ePrint != pType";
}

Unfortunatelly, you haven't provided enough data on the second error, and I can't tell the real reason for it.