You can use Dynamic DNS[0]. Your server contacts a remote server and if the IP address has changed the DNS is automatically updated. You can use a free subdomain[1] and CNAME/ANAME/ALIAS[2] your domain's DNS to point to it.

Edit: Njalla supports dynamic DNS records natively[3].

[0] https://en.wikipedia.org/wiki/Dynamic_DNS

[1] https://freedns.afraid.org/

[2] https://en.wikipedia.org/wiki/CNAME_record

[3] https://njal.la/docs/ddns/