IMO, the best free option is https://freedns.afraid.org/. The biggest downside of that one is that you have to login a couple times a year (IIRC?) to keep it active. I actually still use this even though I have a paid domain, I just CNAME my real domains to the afraid dynamic name. That was easier than changing the config every time I become unhappy with my domain registrar and have to reconfigure everything after swapping.

https://freedns.afraid.org, cloudflare…

I’d recommend getting your own domain name and using a full DNS service that has an API. deSEC.io is free and well supported by common tools like ddclient, but you can also use their API directly with one curl command.

Or you can use your registrar’s DNS servers if they have an API, that would also be free most likely since it would be included in the service.

gandi.net has excellent DNS services included with domains but they’ve recently pumped up their renewal prices and now you have to dig a bit through their offer to find something below $20. There’s .fr, .gdn, .gr, .in and .uk domains (including .me.uk).

I’d recommend an EU country like .fr or .gr if you’re an EU citizen because then their registry data falls under GDPR, plus most EU ccTLD registries are private by default. Otherwise .in also has good whois privacy and is the cheapest.

Duckdns is working for me, wdym, what happened?

Have never used free dyndns services myself. But if I would have to choose one right now I would give ipv64.net a try. Dennis has some excellent (german) YT tutorials and constantly improves his services.