type in run ip a to get interface name
cd /etc/netplan
ls
make sure there is a 01-netcfg.yaml file.  If there ins't, run sudo touch 01-netcfg.yaml
sudo nano 01-netcfg.yaml
add:

network:
  version: 2
  renderer: networkd
  ethernets:
        eth0:
          addresses:
           - 192.168.12.5/24
          routes:
           - to: default
             via: 192.168.12.1
           # gateway4: 192.168.12.1
          nameservers:
            addresses: [192.168.12.1]



press ctrl X to exit, y to save.
run sudo netplan apply to apply new network settings.