Problem with setup of subdomains in flask

Hi I am trying to setup flask blueprints with subdomains. However it doesnt work. i have setup my domain to point to the digitalocean dns servers. I made a created an A record, with @ so i assume that it should cover everything behind the .mydomain.org?

my sample testfile is init.py

from flask import Flask, Blueprint
app = Flask(__name__)
app.config['SERVER_NAME'] = 'mydomain.org'


test = Blueprint('test', __name__)
@test.route("/test")
def testindex():
    return "This is test index page."

app.register_blueprint(test, subdomain='sub')

as described here https://exploreflask.com/blueprints.html

however the result is always ERR_NAME_NOT_RESOLVED.

Anybody have an idea of what im doing wrong? Thanks Best Bjoern