It's probably not relevant but your initial code sample only tries to use http, you then have an automatic redirect to your (non-www) domain. If you initially didn't have all the alternative domain variants in your certificate then your request to the redirect would fail because the hostname wouldn't match the cert.
You should also try a more permissive client connection:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;