Flask server does not recognize my own imported module (ModuleNotFoundError)

In my main Python file, I import another script of mine called helper_1.py (from the subfolder my_helpers) like this:

from my_helpers.helper_1 as h1

However, when I now try to start my server (or deploy it to Heroku), the server will crash with the error notice:

ModuleNotFoundError: No module named 'my_helpers'

I do have a Procfile, requirements.txt, runtime.txt, and wsgi.py.

The content of my wsgi.py is:

from app.main import app
 
if __name__ == "__main__":
        app.run()

MY QUESTION:
Where and how do I have to declare my custom modules (own scripts) so they are properly detected when starting the Flask server?

Everything works fine if I leave out the external reference to my custom module.