First look for a certificate provider who can provide a free SSL/TLS for Server certificate. I have searched and compiled a list of free certificates who can provide a free certificate for QNAP.
I will show you how to get the certificate and install these on the QNAP from startssl.com, with this you can get unlimited free renewals.
Please check the list below:
List of free SSL/TLS certificates:
- https://buy.wosign.com/free ( some time you get untrusted seller ).
- https://letsencrypt.org/ ( Auto install with Plesk ).
- https://www.digitalberg.com/ssl-certificates/ssl-price/ (from top provider).