Issues with apt-get update
Can you try to debug it a bit more with the following commands as root user ?
- dig google.com
- dig mirrors.digitalocean.com
- curl -v http://mirrors.digitalocean.com/ubuntu
- ufw status verbose
My assomption here is that you might not overrides the DNS of DO because they are used to resolv the mirrors.digitalocean.com and others needed stuff (monitoring, ntp ...).
You might want to adding you own nameservers (67.207.67.2 67.207.67.3) with a lowest precedence than the DO ones instead of replacing them.
I don't have a droplet ready right now to check but this is clearly a hint