Hi,
Please refer this document to add device on AWS cloud https://docs.aws.amazon.com/iot/latest/developerguide/register-device.html
-> You should create Root CA cert, client cert and key and then add these certs/key to your device.
This server requires client certification authentication, so you should configure authentication mode =3 with ksslcrypto command
And then, using secured TCP connection to send MQTT packet, ex: at+ktcpcfg=1,3,“xxx.iot.xxx.amazonaws.com”,8883
Please refer this document for MQTT connection on HL series https://source.sierrawireless.com/airvantage/avc/howto/hardware/samples/hl-stnucleo-mqtt-c/
Thanks,