I am glad to read that it works for you now and that you are again a happy user. And thank you for reporting back.

Thanks
Felix

Thank you and indeed. The works :
device.sendMsg("AT+CNACT=0,1\r\n");
readstr = device.waitMsg(200);
log(readstr);

    device.sendMsg("AT+CNACT?\r\n");
    readstr = device.waitMsg(200);
    log(readstr);

    //device.sendMsg("AT+SMCONF=\"URL\",\"broker.emqx.io\",\"1883\"\r\n");
    device.sendMsg("AT+SMCONF=\"URL\",\"mqtt3.thingspeak.com\",\"1883\"\r\n"); // mqtt:// mqtt3.thingspeak.com

    readstr = device.waitMsg(1000);
    log(readstr);

    device.sendMsg("AT+SMCONF=\"KEEPTIME\",60\r\n");
    readstr = device.waitMsg(1000);
    log(readstr);

    device.sendMsg("AT+SMCONF=\"CLEANSS\",1\r\n");
    readstr = device.waitMsg(1000);
    log(readstr);

    //device.sendMsg("AT+SMCONF=\"CLIENTID\",\"simmqtt\"\r\n");
    device.sendMsg("AT+SMCONF=\"CLIENTID\",\"&&&&&&&&\"\r\n");
    readstr = device.waitMsg(1000);
    log(readstr);

    device.sendMsg("AT+SMCONF=\"USERNAME\",\"&&&&&&&\"\r\n");
    readstr = device.waitMsg(1000);
    log(readstr);

    device.sendMsg("AT+SMCONF=\"PASSWORD\",\"&&&&\"\r\n");
    readstr = device.waitMsg(1000);
    log(readstr);

    device.sendMsg("AT+SMCONN\r\n");
    readstr = device.waitMsg(5000);
    log(readstr);

    if(readstr.indexOf("ERROR") == -1)
    {
        Serial.println("We have a MQTT connection");
        
        break;
    }
    else {Serial.println("SMCONN ERROR " );}

you most likely are missing parameters USERNAME and PASSWORD for a successful connection.

Thanks
Felix