Can you try wss://my-app.herokuapp.com/socket instead?
I am not sure if this will work but the default url defined in the Endpoint module is "/socket" …

Did you update the allowed incoming url’s in your configuration?

I ran into a similar problem and documented some of the different things I tried to fix it in this SO post.

I never got it to work properly and resorted to hard-coding my socket variable before pushing to production. If there’s a right way to accomplish this I’d love to know.

heroku gives an error about setting the check_origin parameter for me that did it

url: [scheme: “https”, host: “myapp.com”, port: 443],
check_origin: [
“https://myapp.com”,
“https://www.myapp.com”
],

I have url: [scheme: "https", host: "secret-sea...herokuapp.com", port: 443] within my Endpoint config in prod.exs. Not sure if that is what you are referring to or not.