Let’s Encrypt doesn’t issue certificates for IP addresses, though some CAs do.
You could buy a domain.
You could get a free domain.
You could use a dynamic DNS service to get a subdomain.
In particular, would Synology DDNS work for you?
https://www.synology.com/en-us/knowledgebase/DSM/tutorial/General/What_Is_Synology_DDNS_Service