This is a great guide and here is an important update for those who wish to use it.
If you use the latest LTS release of Ubuntu server (18.0.4) , you will have FreeRadius 3.0, and there is an issue in the PAM implementation, namely it's missing a symbolic link. After a bit of troubleshooting and Googling, I stumbled upon this:
The solution described to manually add the symbolic link via the mentioned command and restart the FreeRadius service solved the issue and I have now RADIUS working on my new Ubuntu Server.
sudo ln -sf /etc/freeradius/3.0/mods-available/pam /etc/freeradius/3.0/mods-enabled/pam
This doesn't survive reboot, so depending of your implementation you will need to re-enter that command after each restart or find a way to automate it.