How to use mongocxx connection from different class or method in QT environment?

Question

As a Scala/Java girl sometimes I am totally losing my mind in C++ / QT.

I have very simple thread class like :

 mongoUpdate::mongoUpdate(QObject *parent) :

    QObject(parent)
{

        mongocxx::options::client client_options;
        mongocxx::instance inst{};
        mongocxx::uri uri("mongodb://xxxx");

        mongocxx::client client(uri);

//        db = client["aces"];
//        coll = db["aces"];

        qDebug() << "PARENT   Mongo Connected" << endl;
        //     MONGO

}

void mongoUpdate::send_mongo(QImage m_qimg)
{

 Write the mongo m_qimg. bla

}

Problem is send_mongo part is not aware I have a connection.

How can I make a mongo connection to be know and usable by send_mongo thread ?

Edit 1

I create a singleton mongo instance as described in : instance_management.cpp

Now it is connecting and able the insert data. But when coll.insert_one called main GUI thread freezes even the mongoUpdate running in a different thread with movetothread way in QT.

Even we are using the mongoupdate in a different thread why it's blocking mainGui ?