Then usually, the only other issue would be, if the port is blocked.
So if you try putting the REST API for $metadata in the URL and it does not return anything, then it is most probably due to port being blocked.
So check and ensure that the $metadata xml is accessible from where Canvas is. If it is accessible, whatever URL you have used in your browser to access that, should be the same as what is inside your restUri in instances.json.
For example, if you typed: https://server-prod:12345/api/v1/$metadata in the browser and it returns the XML, then the instances.json should have an entry like: