Cumulocity SmartREST GET Template using MQTT

Question

I am trying to receive a response using a custom GET template implemented on the Cumulocity UI however I'm having some trouble. I have defined my message template like so:Message Template and my response template like so: Response template.

At the moment when I publish to topic 's/uc/template1' (where template1 is my X-ID) with the payload of '999, 12345'(where 12345 is my External ID), no response is received and no error is found. The speeds are known to exist in the object because when I request a GET command to {{url}}/inventory/managedObjects this responds with:

            "speed": {
            "1": {
                "unit": "m/min",
                "value": 1
            },
            "2": {
                "unit": "m/min",
                "value": 1
            }

I've also followed this post where @TyrManuZ suggests to add c8y_Global: {} to the application. I have done this through a PUT request to {{url}}/inventory/managedObjects/{{deviceId}} on Postman and can verify that c8y_Global: {} exists there.

Is there something I am doing wrong, and if so what is the correct way? Any help would be greatly appreciated!