The letsencrypt website assumes you are securing a public hosted website. However, I have a Buffalo Linkstation NAS which is accessed via a local webpage on a local IP Address. It has a setup window asking for a certificate and key file which is supposed to be written to the root folder. Whenever I access the admin webpage I get a screen saying the webpage is not secure and I have to click several links to get access. I was hoping to get a certificate and key file you be able to access the admin page via https. I am not sure how to proceed since it seems the certbot method does not apply to my needs?
If you don't have a public domain, then you will have to issue a private cert for that RFC-1918 IP address.
You can provide the cert with a very long life.
But you will have to trust that cert [or the issuer] on all devices that will access it securely.
If you do have a public domain, then you can simply use DNS validation to obtain a certificate for any valid FQDN from it [despite the IP address(es)].
You can also get a cert for an IP Address but it must be a public IP. You must use the shortlived profile: Profiles - Let's Encrypt and an ACME Client that supports that