To expand on this, Phoenix enforces same origin for websockets by default, (check_origin: true endpoint config). By default, we will enforce the same origin as configured in your :url, but you can also provide a list of hosts if you have other needs. For example:
config :my_app, MyAppWeb.Endpoint,
check_origin: ["//example.com", "//another.com"]
For applications generated with phx.new >= 1.6.3, the config/runtime.exs will use a PHX_HOST environment export if available to set the endpoint url host, and fly.toml which is set to myapp.fly.dev when you run fly launch, and can be customized as you add your custom domains. Glad you got things sorted and hope that helps!