I figured out the issue, which is with mongoose new versions i.e >4. If you use mongoose > 4 and use the new .then command in the controllers, its doesn't work. I had to use an old version of mongoose i.e. "mongoose": "~3.8.19", and used . success instead of .then in the controllers, and it worked fine.
Here is the live version on Heroku.
https://user-crm-420.herokuapp.com/
I still would like to know why new versions of mongoose are not working with .then, any explanations.