Networking
Firewall basics
A new server accepts SSH. Everything else is yours to open.
Default
A network firewall in the portal is not a substitute for a host firewall on the machine. Plan for SSH (22) from addresses you trust, and 80/443 if you serve websites. Deny other inbound traffic.
Ubuntu
ufw allow OpenSSH, ufw allow 80/tcp, ufw allow 443/tcp, then ufw enable. Confirm you still have an SSH session before you enable it. IPv6 rules should match IPv4 if the location gave you an AAAA address.