Have you checked the console in Mendix and/or Chrome via the Chrome DevTools? If anything on the client side goes wrong you might see some messages over there. First suggestion would be to add the <script> tag to your index.html to make sure it is loaded the first time your app starts, this way you only have to add the <div> tag to the HTML snippet widget!
I got it to work!
So I inspected the standalone webpage I created and copied the full iFrame code into the HTML Snippet widget.
Thank you for all your help Freek Brinkhuis, your debug techniques led me to the idea.