It's always DNS.

There are other services which should work for you as well. I use one which never requires a regular check in. They even ofter a command line curl or other methods to send your updated IP to it. Think Home Assistant, via a curl or if you have a router with Freshtomator or other which supports such curl, it will work as well.

https://freemyip.com/

I've been using them for a few years now and never ever had an issue with uptime or otherwise. My use is just as a backup in the event my VPS goes down which hosts Pangolin (VPS).

Others such as noip.com and such do work well also.

No clue. Don't use it myself. I am able to access the website so I would assume the rest is up and working.

Your ISP may be blocking access (many ISP's do not allow hosting things on home connections so check your TOS.) Why they would block access to the website I have no clue but they could see allowing it as allowing people to host a service.

Disconnect your phone from wifi and see if you can access DuckDNS. You can also do some troubleshooting and testing https://www.golinuxcloud.com/how-to-check-if-isp-is-blocking-website/

Also depending on your router if you can setup DNS over TLS or DNS over HTTPS and use two different DNS providers not controlled by your ISP.

If you're using an ISP provided Modem/Router you should probably consider setting up a proper firewall router and get just a modem or put the modem/router in bridge mode so you don't double NAT.

Hey you got it! On mobile data I can access duckdns. That's weird. I've been using my ISP for a year and haven't had issues with duckdns so far. thanks for the tip!