Flask backend working but React frontend not - Heroku deployment - reactjs

Question

I'm working on getting a three-tier application running on Heroku. The backend is a Flask API with a React frontend. All works well locally and (ideally) I am trying to run both on the same server.
On deployment, I can access (public) API routes. However, at the index route I receive:
applicationName.herokuapp.com/:1
Failed to load resource: the server responded with a status of 404 (NOT FOUND)
My directory setup is:
Deployment
+-- __pycache__
+-- venv
+-- .env
+-- .gitignore (currently empty)
+-- app.py
+-- config.py (contains python dictionary with details for mail/database config)
+-- database.db (sqlite, 1 entry)
+-- profile
+-- requirements.txt
+-- frontend
| +-- build
| +-- node_module
| +-- public
| +-- src
| +-- package.json
| +-- package-lock.json
| +-- webpack.config.js
requirements.txt was generated via pip freeze prior to deployment. frontend/build was generated via npm run build, again just prior to deployment.
I've tried various approaches and tweaks to both server and client with no success. I've seen approaches that involve moving build and/or package.json to the root directory, but this did not work. I feel like I'm missing something fairly simple but I cannot for the life of me figure it out.
Select code excerpts include:
app.py
app = Flask(__name__, static_folder='/frontend/build', static_url_path="/frontend/public/index.html")
#app.route('/test')
#cross_origin()
def test():
return 'success' #*applicationName*.herokuapp.com/test works correctly
#app.route('/')
#cross_origin()
def serve():
return send_from_directory(app.static_folder, 'index.html')
if __name__ == ("__main__"):
app.run(host='0.0.0.0')
procfile
web: gunicorn app:app --log-file=-
frontend/package.json
{
"name": "frontend",
"version": "0.1.0",
"private": true,
"proxy": "https://*applicationName*.herokuapp.com/",
.env
FLASK_APP=app.py
FLASK_ENV=production