Ahh perfect! For a tip for the future, cause I know that this can be a bit confusing with all of the different APIs, and no one size fits all solution.
External Collections are intended to load a list of records returned in an API response. in order for that to work, those records need to be in an array. An array sits between square brackets [array]. So you need to use the results key and sub child keys to get to that[array] list of data you are looking to use in your app. Sometimes there are multiple arrays, or child keys that are arrays in the array. You will need to find the correct one for your use.
Here is a break down of your APIs response screenshot, and how I identified those keys! I hope this helps you and anyone else that may be frustrated setting up the External Collection GET ALL requests.
