Why Im getting CORS error when using custom domain Auth?

I decided to use the custom domain Auth since getting outside of my domain for the login created cross-origin privacy issue.

After creating the sub-domain and setting up Auth0 accordingly, Im getting CORS error from my domain.com to api.domain.com. The login is successful in the Auth0 Logs but somehow my browser is saying there is no headers Access-control-origin. In my console the OPTIONS header is successful but the POST is denied.

The issue is I always had the header there (Nginx) and it was working perfectly before I switched to custom domain login. I was on the impression that changing the login domain has nothing to do with my backend service. Any idea why?