What is the reason you do not want a domain? it is not that DNS-domains are that expensive these days. The cheapest option I found is .ovh (which is one of the major cloud-providers in France), which is 3 euro / year (+VAT). You can then put as much hosts or subdomains under it, and it supports dynamic IP.
Agreed, .ovh is not the most “professional” looking domain, but it depends on what you want to do. If your goal is simply to have something for yourself / family / friends, then this is good enough.
BTW. Having your own domain for a nextcloud instance has additional advances: you can get a real https/tls certificate from letsencrypt, and -if you put a reverse proxy in front of your NC- it shields you from people who just scan the complete IP-space of the internet but who do not know your domain.