As of April 2019, it seems fresh server installations of Debian do not attach properly to mirrors. This article explains how to manually add a mirror. Please note it’s for Debian Stretch, do cat /etc/debian_version and ensure you’re working with version 9.
nano /etc/apt/sources.list
Add official repo:
deb http://ftp.us.debian.org/debian stretch main contrib non-free
Update:
apt-get update
Now you can do stuff such as:
apt-get install openssh-server
Reference:
https://linuxhint.com/how-to-add-a-package-repository-to-debian/