Based on the discussion here: https://forum.vyos.io/t/dynamic-dns-not-wollowing-web-options/12309 it seems necessary to note that setting the web-options on a given interface is not sufficient for determining the IP address when behind NAT.
I've added some additional detail, which I think will make that more clear, as well as listed the commands as required to set up DDNS behind NAT.
Further I updated the section on RFC2136 to accurately show address instead of interface