Well I tried to find where it builds the request string and sets curl parameter and am unable to. But it was easy to find noip's URL.
So I just added it to my monitor and am directly sending addr updates there.
Indeed it's pretty easy, most of ddns's complexity is for being able to support multiple devices and noip services and LuCI.
I think OpenWRT PPPoE support should be improved. There are many ppl having routing issues, ping and traceroute getting permission error, and the change on interface's name while not accepting original interface makes harder to support for other softwares like ddns.
After installing 21.02RC1 I started having the same problem, and I fixed it be reverting this change from last October: https://github.com/openwrt/packages/pull/13183 and added some comments there but have not got a response yet.