Think about it, how is this supposed to work, what did you forget?
curl -v -u root:enigma2 --tlsv1.2 https://localhost:9091/transmission/rpc
-->
An SSL connection without an SSL certificate on the box has never worked.
Try this:
curl -k -v -u root:enigma2 --tlsv1.2 https://localhost:9091/transmission/rpc
Or buy a domain and get an SSL certificate for each box with Let's Encrypt, or use a self-signed SSL certificate and suppress the error message.

