EC2 AMI built from Ubuntu 18.04 with ubuntu-desktop installed is unreachable

I also had this exact same problem with Ubuntu 18.04 on AWS. This answer resolved it for me:

The correct way to do it is to use an empty /etc/NetworkManager/conf.d/10-globally-managed-devices.conf that will overshadow/overwrite the one in /usr/lib – solsTiCe Jan 2 '17 at 13:41

This answer is from this Ask Ubuntu question: https://askubuntu.com/questions/838865/network-manager-refusing-to-manage-wired-interfaces

I wanted to post this as a comment (because the other answer is correct) but I can't because I don't have enough reputation.

Anyway the exact command I entered was:

touch /etc/NetworkManager/conf.d/10-globally-managed-devices.conf

And I was then able to SSH in.

This was taken from this comment 6 on this issue: https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1638842/comments/6