Database connection failed error on Moodle instance - enable service to make it start automatically on reboot

The problem was that I had not "enable"d the tunnel service (details in a previous post) which was tunneling the database connection over ssh - so, on server reboot, the tunnel service was not restarting automatically. We need to do 

sudo systemctl enable name-of-our-service

to make it start automatically on reboot, and then

sudo systemctl start name-of-our-service

to actually start it the first time.