'Heroku deployed but endpoints are not working

I can deploy a node.js/React web app successfully to Heroku which uses MongoDB, all pages that do NOT pull in data load fine.

However when I send a request to my endpoints, for example /signin , I get

at=error code=H12 desc="Request timeout" method=POST path="/requests/sign

and in console

503 (Service Unavailable)

To add, I have added the connection variables to the heroku config variables and have also added 0.0.0.0/0 to the whitelist in Mongodb.

Can someone please help.

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