How do I link my app to a custom subdomain?

I have added both my domain CNAME and TXT records as provided and
Created a subdomain but this still does not work.

Both my website and domain hosts (they are different) can’t seem to figure out how to do this.

Current error reads:

Forbidden

You don’t have permission to access this resource.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

Really appreciate your support.