Could not retrieve WSDL

I installed the Dynamics 365 Integration plugin and I’ve been following the AlexaCRM configuration instructions;
https://docs.alexacrm.com/wpcrm/configuration/

But I’m currently getting the error message:

Unable to connect to Dynamics 365: Could not retrieve WSDL at https://crm.gasair.com.au:444/XRMServices/2011/Organization.svc?wsdl. Check whether you selected the appropriate deployment type and that CRM URL and credentials are correct.

Our Dynamics is setup to be a Internet-facing deployment, and I’ve triple checked the URL and credentials. I can also see that there is a xml file at the WSDL URI.

How can I fix this error? Is there something I’ve missed?

Thanks heaps for your help in advance.