Although a PING from the PC terminal shows you the correct IP, in OPNSense I don't see it and instead I find the error in the log
The configuration is this:
Services: DuckDNS
Username: The ID in DuckDNS
hostname: MyHostName.duckdns.org (MyHostName created in Duckdns)
Check ip method: Interface
Interface to monitor: WAN
Check ip timeout: 10
Force SSL : Off