How to publish msgs to MQTT broker in the core when it is in disconnected mode?

I have a GG core running on a Pi and connected to a device via ethernet. They will be located in a field where there will be multiple internet disconnections for 10 to 20 minutes each. I want the MQTT connection between the core and the client device to continue to work. Here are my questions:

1. How to publish/subscribe to MQTT in the core in the disconnected mode?
2. How do you configure the max time for the credentials to be cached by the core?
3. How to set a lambda function to work on the core in the disconnected mode as well?

The IPDetector in the core detects the static IP address that I set for the GG core but when I tried it in the code below at the endpoint URL, it was not working. I tried adding SSL:// in the beginning for both the public and the static IPs but not working. I want the code to work with my static IP address for the core since it will be the only connection between the core and the client device when there is no internet. I also want to have a subscriber at the core that does not have to check the internet, otherwise, it will not work when there is not internet.

Below is the code I tried for the publisher on the client device:

# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
# SPDX-License-Identifier: MIT-0

import time as t
import json
import AWSIoTPythonSDK.MQTTLib as AWSIoTPyMQTT

# Define ENDPOINT, CLIENT_ID, PATH_TO_CERTIFICATE, PATH_TO_PRIVATE_KEY, PATH_TO_AMAZON_ROOT_CA_1, MESSAGE, TOPIC, and RANGE
ENDPOINT = <PUBLIC_IP/STATIC_IP FOR THE CORE DEVICE>
CLIENT_ID = <MY_CLIENT_THING_NAME>
PATH_TO_CERTIFICATE = <PATH_TO_CLIENT_CERTIFICATE>
PATH_TO_PRIVATE_KEY = <PATH_TO_CLIENT_PRIVATE_KEY>
PATH_TO_AMAZON_ROOT_CA_1 = <PATH_TO_AMAZON_ROOT_CA1><
MESSAGE = "Hello World"
TOPIC = "clients/TEST/hello/world"
RANGE = 20

myAWSIoTMQTTClient = AWSIoTPyMQTT.AWSIoTMQTTClient(CLIENT_ID)
myAWSIoTMQTTClient.configureEndpoint(ENDPOINT, 8883)
myAWSIoTMQTTClient.configureCredentials(PATH_TO_AMAZON_ROOT_CA_1, PATH_TO_PRIVATE_KEY, PATH_TO_CERTIFICATE)

myAWSIoTMQTTClient.connect()
print('Begin Publish')
for i in range (RANGE):
    data = "{} [{}]".format(MESSAGE, i+1)
    message = {"message" : data}
    myAWSIoTMQTTClient.publish(TOPIC, json.dumps(message), 1) 
    print("Published: '" + json.dumps(message) + "' to the topic: " + TOPIC)
    t.sleep(0.1)
print('Publish End')
myAWSIoTMQTTClient.disconnect()