First of all, does it work when using curl ?
I don’t know how you have configured nginx, but I’m guessing that unless you are doing tcp-proxying directly, you won’t be able to talk https to the cluster service. Perhaps nginx doesn’t know how to talk https to the upstream server. The normal thing is to do ssl termination in nginx, if nginx is going to be there anyways.
Potentially, nginx is trying to make an HTTP request to the HTTPs server, or your client is trying to make an HTTPs request to the HTTP nginx. That is probably happening if you are doing http-proxying instead of tcp proxying.