What does your nginx config look like for ssl? It should specify a certificate and key file - that certificate subject needs to match your fully qualified domain name (fqdn). Certificate can have subject alternative names (SAN) for other names and even IP addresses.

For instance, you could have a single certificate for foo.bar with a SAN for just foo and an IP SAN for 192.168.1.30.

Certificates also need to be signed by a certificate authority (CA), and in order for your browser to visit https://foo.bar/ without a warning your browser must trust that CA.

If you did a self signed cert, this is most likely the problem you’re running into.

It’s important to know that your communication is still encrypted because of SSL, but since your browser doesn’t trust the CA (or the subject doesn’t match the FQDN) the browser will say it’s not secure.