My router was successfully using the DDNS script for DuckDNS in 19.07, but after updating to 21.02.0 RC1 it stopped working. Tracing the issue showed that this commit broke it: https://github.com/openwrt/packages/commit/5b362932a7df5ce44ca3a8f4cf1d2b225ca8d5ae
which is discussed here https://github.com/openwrt/packages/pull/13183
I attempted to contact the person who created the pull request (@mrjoel) to ask him to try a different fix for the issue he experienced, but he has not responded. This issue was also discussed here: DDNS not working with PPPoE interface - #3 by Hikari but I didn't get a response to my request to revert there, so am trying again in this new thread.
Please consider this a request to revert the commit linked above for the next release.