Not having any experience with stunnel, this will take a significant amount of my time to setup.
Is there any reason to believe that the certificates provided / generated by Amazon is incorrect?
These certificates work using their python example program to connect to the broker.
When uploading them to the BX3105 the module confirms “OK” after each certificate. So I am confident that the uploading process is correct.
Can you confirm that the BX3105 with FW 2.6.3 has been tested to connect to an MQTT broker using the TLS mode?