I am sorry this hasn’t been answered sooner. We have had little capacity to update our tutorials lately, so thank you very much for your feedback.
First of all, I can see that the hostname is wrong, it should be a15nxxwvsld4o-ats.iot.eu-west-1.amazonaws.com. (I will change this in the code example as well.)
Second, the AWS root certificate that you are looking for should be the listed MIC_ROOT_CERT.
Hopefully with the change of host it will work seamlessly. Once again sorry for the late reply and thank you for the feedback.