Send client member-function to SLOT() macro

Question

I've currently studied QT and have to face the problem of passing member function pointer to SLOT macro. Here is the part of code:

main.cpp:

#include <iostream>
#include <QApplication>
#include "vMainMenu.hpp"
     
int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    try{
        MainMenu* menu = new MainMenu{};
        menu->show();
    }
    catch(const std::exception& e)
        std::cerr << e.what() << '\n';
    return app.exec();
}

vMainMenu.hpp:

#pragma once
#include <QDialog>
#include <QtWidgets>
#include "vMainMenu.hpp"

class MainMenu : public QDialog
{
    Q_OBJECT

public:
    MainMenu(QWidget *parent = nullptr){
    m_Layout = new QVBoxLayout{};
        showMainMenu();
    };
    virtual ~MainMenu();

private slots:
    void showMainMenu(){
    addButton(tr("&One player"), &MainMenu::showOPMenu);
    addButton(tr("&Network"), &MainMenu::showNWMenu);
    setLayout(m_Layout);
    m_Layout->update();
    };
    void showOPMenu(){};
    void showNWMenu(){};

private:
    void addButton(const QString& str, void(MainMenu::*fn)()){
    QPushButton* btn = new QPushButton{str};
        // more button config ...
    m_Layout->addWidget(btn); 
        QObject::connect(btn, SIGNAL (clicked()), this, SLOT(fn)); // this metod doesn`t work
    }
    QVBoxLayout *m_Layout;
};

CMakeLists.txt:

    cmake_minimum_required(VERSION 3.8)
    project(proj)
     
    set(CMAKE_CXX_STANDARD 17)

    find_package(Qt5Core REQUIRED)
    find_package(Qt5Gui REQUIRED)
    find_package(Qt5Widgets REQUIRED)
     
    QT5_WRAP_CPP(MOC_Files vMainMenu.hpp)
    add_library(forms SHARED${MOC_Files})
    
    target_link_libraries(forms Qt5::Core)
    target_link_libraries(forms Qt5::Gui)
    target_link_libraries(forms Qt5::Widgets)

    set(SOURCE_FILES main.cpp)
    add_executable(proj ${SOURCE_FILES})
    
    target_link_libraries(proj forms)

Is there a method to modify addButton function to receive a pointer to the nonstatic member function (or some other way) and push it to SLOT macro. I tried to send it as a parameter. The program has been built, but when it runs, it throws an error:

QObject::connect: Parentheses expected, slot MainMenu::fn ...

I understand that it tries to interpret 'fn' as MainMenu::fn, but Is there a method or some kind of abstraction over member-function to read by SLOT macro?

I have already read this QT SLOT: Pointer to Member Function error post but when I tried to use that method, the program has been built, but when it runs, it throws an error:

No such slot MainMenu::callSlot(fn)