API calls to a Flask server with the local port shared with pinggy.io

I have my Falsk server running locally on port 5000;
I share the port with Pinggy and can access my APIs (i.e. http://rnpni-2-39-42-39.a.free.pinggy.link/items) using Postman, getting back some JSON data as expected.
Then I want to make the same API calls using Flutterflow.
If I test a public API (i.e. https://jsonplaceholder.typicode.com/todos/1) in the FluttefFlow app, it works, but when using the Pinggy address I get back as response 200 and the Pinggy Caution Page as data instead the JSON I was expecting.

Any idea...?