Installed mongodb on Ubuntu 18.04 LTS, mongod service enabled to start automatically on boot. Configured mongodb.conf to start on 127.0.0.1 and host IP. Could start/stop service with systemctl but failed to start on reboot. If bindIp changed to 0.0.0.0, would work. Unlike most of my machines, this one runs NetworkManager instead of the traditional ifupdown/networking stack.
After some experimentation this worked for me:
Changed mongod.service under /lib/systemd/system to use "After=network-online.target" instead of "After=network.target". This results in systemd waiting until networking is completely up before attempting to start mongod (something "network.target" does not require, see https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/).