I am running Filemaker Server 17 on an iMac (from 2010 or so), on which I now make a database publicly accessible via FIlemaker Webdirect (https://126.96.36.199/fmi/webd) - for this page I would need an SSL certificate so that the database can be accessed from all browsers. Can I get such a certificate from let´s encrypt?
short answer: No, because you use an ip address.
Better answer - yes. Use a domain with that ip address as A record and change the link, so the domain is used.
Then you can create a certificate.