You should try the appInfo config like:

{
  ...
  apiDomain: "YOUR CUSTOM API DOMAIN WITHOUT A PATH",
  apiGatewayPath: "/dev"
}

And see if this works. This assumes that you query "https://yourcustomdomain.com/dev/auth/signup POST" via postman. If on the other hand, your custom domain points to /dev/*, then you don't need the

apiGatewayPath