You have IPtalbles default on Linux distro servers. You can configure it to restrict and allow IPs / packets. To harder the security, SSH ports, strong passwords, FTP details, etc need to be changed from the defaults.
Further implement CSF Firewall- available for Linux- simple and easy to install and configure. There are other tools like Brute Force Detection tools,etc which add to the security.

A managed server usually comes with the Security tools to harden your server. Hope this helps