1. Services > DNS > Add DDNS Interface
Interface: 'eth0' - or other WAN interface
Web: '-'
Service: 'custom' 'https'
Hostname: '<domainName>' - not FQDN
Login: '<domainName>'
Password: from DuckDNS.org > Install > openwrt > option password
Protocol: 'zoneedit1'
Server: 'www.duckdns.org/nic/update?hostname=<domainName>&wildcard=NOCHG&mx=NOCHG&backmx=NOCHG' - replace <domainName>2. Check the result (CLI on router)
show dns dynamic status