Connecting to mongocxx in multiple slots

I'm using the mongocxx driver in a project which is powered by Qt 5.11. Because there are only a few articles about it on the internet, I couldn't find a solution to my problem: I'm trying to connect and get data from my Mongo database but multiple times with slots. Here's an example of what I'm trying to do:

#include "mainwindow.h"
#include <bsoncxx/builder/stream/document.hpp>
#include <bsoncxx/json.hpp>

#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>

MainWindow::MainWindow() : QMainWindow() {
    mongocxx::instance *inst{}; //shouldn't be done twice
    mongocxx::client conn{mongocxx::uri{}}; //this neither
    bsoncxx::builder ::stream::document *document{};

    //doingStuffWithoutMongo();
    //then
    LoadClients();
    GetAndSetBanks();
}

void MainWindow::LoadClients() {
    //doingStuffAndGettingDataFromMongo();
    auto collection = conn[awesomeDatabase][awesomeCollection]; //use of undeclared identifier 'conn'
}

void MainWindow::GetAndSetBanks() {
    //doingStuffAndGettingDataFromMongo();
    auto collection = conn[awesomeDatabase][awesomeCollection]; //use of undeclared identifier 'conn'
}

I need the identifier 'conn' in both of my slots but I can't call it more than once. I also tried to declare it in my mainwindow.h file but i'm getting this error:

Maybe I'm declaring something wrong, I don't know, I'm not at ease with this language yet, if somebody could help me, I would be glad to accept her/his answer.