Express API calls that connect to Mongo gives 503 when hosted in Heroku. When I call a route that just res.sends text it works just fine

Question

Express api hosted in Heroku. Any route that connects to the database returns a 503 (Service Unavailable). However a route that just simply returns text, such as res.send("Hello") works just fine. My connection to my DB is correct, it all works just fine in localhost.

// Doesn't work (Gives 503)
router.get("/", (req, res) => {
  Quote.find().then((items) => res.json(items));
});

// Works just fine
router.get("/quote", (req, res) => {
  res.send("Hello")
});