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?
Try this...
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
Code
ken@openmediavault:~$ host myip.opendns.com resolver1.opendns.com
Using domain server:
Name: resolver1.opendns.com
Address: 208.67.222.222#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?