Any DynDNS client for current iOS for usage with nsupdate.info?
nsupdate.info should work with any dyndns2-compatible client

You usually need it so your router (or other dyndns2-compatible client) can update a hostname, so you don't need to know or find out your current IP address

You also can do quite a lot of scripting with the Shortcuts app (https://support.apple.com/guide/shortcuts/welcome/ios) which is part of iOS (you may need to download it from the Store in case you removed it in the past). There are actions to call any URL and to process JSON results.