that’s the errormessage you’ll get when using selfsigned certificates like you do.
you need to tell your browser to accept this certificate (and then it’s gonna be valid for 3 months, i think)
I suggest to use letsencrypt/certbot to avoid these message. Because every self signed certificated will be detected by any browser and has to be acppected in the browser.
I can support this. I have letsencrypt certificates on all my servers more than one year now, and never had SSL issues again. Works perfect. I use the getssl script to create and update my certificates: https://github.com/srvrco/getssl