I researched more and found these;
My default network configuration is here: /etc/systemd/network/50-default.network
Main IP address, gateway and other things are resides in that file.
This explains strangeness that even /etc/network/interfaces not exist and /etc/netplan directory is empty, system is still running without problem.
This page helped me to add new IP addresses:
I created 50-cloud-init.yaml file and fill it with these informations,
Then run netplan apply and it's working now.