Something isn't right here.. especially with duckdns logs showing your IP changing constantly. Is your laptop/client computer behind a VPN, when you check your duckdns account?
SSH your server... and execute the following the command
You should get an output something like this...
It should give you an output something like this
ken@openmediavault:~$ host myip.opendns.com resolver1.opendns.com Using domain server: Name: resolver1.opendns.com Address: 220.127.116.11#53 Aliases: myip.opendns.com has address YOUR.IP.HERE ken@openmediavault:~$
Does that IP address (where I put YOUR.IP.HERE) match your client machine (whatismyip.com) and what is in duckdns?