Upgrading Ubuntu 20.x -> 22.x killed my network

So the answer above didn’t work for me. What I ended up doing was creating a new YAML file at /etc/netplan/config.yaml with the following content:

network:
  version: 2
  renderer: networkd
  ethernets:
      eth0:
          addresses: [ "<droplet IP address>/24" ]
          routes:
            - to: default
              via: <droplet gateway IP address>
              on-link: true #only needed if the droplet is on a different subnet

If your droplet gateway is on a different subnet (i.e. more than just the last set of numbers in the IP address and gateway IP address are different), you need to include the “on-link” line.

Save the file and then run this command:

netplan generate

followed by

netplan apply

That will set the static IP and gateway address and make it persist past reboots.

If you need to temporarily add the static IP address and gateway, use these commands:

ip addr add <droplet IP address> dev eth0
ip link set dev eth0 up
ip route add default via <gateway IP address> onlink dev eth0

This will give you connectivity in order to run updates and install additional packages, but that connectivity will not persist on reboot.

I hope this helps someone.