Hi @anyaitsystems, welcome to the community forum 
Unfortunately Let's Encrypt doesn't issue certificates for bare IP addresses, only domain names. You'll need to register a domain name in order to get a Let's Encrypt certificate.
I think there are some "Dynamic DNS" providers that may allow you to get a subdomain of a registered domain name for your IP address without paying but I can't make any recommendations personally. Also beware if you go down this route because sometimes these providers run into rate limit problems with Let's Encrypt if they aren't registered with the public suffix list.
Hope that helps!