SSL/TLS certificate was installed on IP address, or Plesk was secured with a Let's Encrypt certificate.
While trying to reach the IP at https://203.0.113.2, or log into Plesk over https://203.0.113.2:8443, the following warning still appears:
Your connection is not secure
Why does it happen?
It is expected behavior. The SSL certificates are usually associated with the fully qualified domain name, not an IP address.