There have been a number of threads concerning this topic which I have read through. Unfortunately, none of them deal with the specific issue I am experiencing.
The board I'm using is the Digi XBee Cellular LTE Cat 1 Development Kit and I have created a thing, attached a policy and downloaded / renamed the certificate files as described in the Digi Micropython Programming Guide. The policy attached to the thing grants all the privileges described in the Guide.
I've tested the certificates with the Python 3 script (aws_https_pc.py) described in the Guide and get the expected response.
However, after I upload the three files (/flash/cert/aws.key, /flash/cert/aws.crt, /flash/cert/aws.ca) with the File system Manger tool, copy and paste the Micropython version of the "test connection" code with my parameters and run with CTRL+D, I receive an ETIMEDOUT error. This occurs using both destination ports 8443 and 8883.
I notice in previous threads that users have been having problems with ECONNREFUSED errors, indicating that the server is rejecting the connection attempt. In my case, it appears that a connection is not even being attempted.
Several other points:
a. I've tried using AWS endpoints both with and without the "-ats" suffix and get the same results.
b. I can connect successfully to the AWS MQTT broker using a Python 3 script running on my PC and also using Node-Red.
c. The Digi board successfully connects to a broker I've set up on my company's server using Mosquitto.
Any help with this issue will be greatly appreciated.