Mongodb query doesn't post or put data in deploy

Question

I'm developing a simple forum website. It has a react frontend + node backend, with mongodb cloud as database and AWS Cognito for signup/ verification/ sign in. When a user signs up: 1. create a user in Cognito user pool 2. create a user in mongodb users collection 3. update the user's userID entry inside Mongodb users collection using the User ID (Sub) from Cognito.

My user schema:

const userSchema = new mongoose.Schema(
  {
    userID: {type: String},
    username: {type: String, required: true, unique: true},
    email: {type: String, required: true},
    issuesPosted: [{ type: Schema.Types.ObjectId, ref: 'issue' }],
    issuesFavorited: [{ type: Schema.Types.ObjectId, ref: 'issue' }],
    commentsAdded: [{ type: Schema.Types.ObjectId, ref: 'comment' }],
    profileImageUrl: { type: String, default: '(a link to default user pic in S3)' }
  }
)

Everything works in dev. However in deploy, I noticed after signing up in my forum website, it ONLY creates a new user in aws cognito, but NOT in mongodb. Same problem with sign in: it doesn't fetch any user data from mongodb. In a page where it's supposed to show existing posts which I created during dev as mock users, it also doesn't show those data in deploy.

Could anyone help me?

Below is the sign up page:

//a bunch of imports

export default function Signup() {
  const [username, setUsername] = useState('')
  const [email, setEmail] = useState('')
  const [password, setPassword] = useState('')
  const [verifyProcess, setVerifyProcess] = useState(false)
  const [OTP, setOTP] = useState('')
  const navigate = useNavigate()
  const { authenticate, getCurrentUsername, getCurrentUserId } = useContext(AccountContext)

  // when user clicks sign up
  const onSubmit = (e) => {
    e.preventDefault()

    // setups for sign up in Cognito

   const verifyAccount = (e) => {
    e.preventDefault()

    // setups for verification in Cognito

    user.confirmRegistration(OTP, true, async(err, data) => {
      if (err) {
        alert(`Couldn't verify account: ${err}`)
      } else {
        try{
          // create user in mongodb (the problem arises here)
          await axios.post('/api/signup', { username, email })

          // authenticate the user in cognito to get the session (works fine)

          // update userID in mongodb (the problem also here)
          const realUsername = getCurrentUsername()
          const userID = await getCurrentUserId()
          try {
            await axios.put(`/api/useridupdate/${realUsername}`, {
              userID: userID
            })
          } catch (err) {
            console.log('Error updating userID:', err);
          }

          alert('Account verified, please log in')
          navigate('/signin')
        }catch(err){
          console.error('Error creating user in MongoDB:', err)
        }
      }
    })
  }

  // const resendVerifyCode = () =>{}

  // return (page components)
}

Below are the logs after user sign up in deploy:

HTTP  01/07/2024 22:06:07 ::1 POST /api/signup
HTTP  01/07/2024 22:06:07 ::1 Returned 200 in 1 ms
HTTP  01/07/2024 22:06:07 ::1 PUT /api/useridupdate/leahalt
HTTP  01/07/2024 22:06:07 ::1 Returned 200 in 1 ms

It seems that it's creating a user in mongodb and updating the userID entry successfully. What could be the problem?

I have the mongodb connection data in .env. When I noticed the problem, I tried directly putting the connection data in my server.js to test the connection in deploy, yet this still failed. Therefore I don't think it's a problem of the .env file.

I've deployed on both local machine and EC2 instance, similar result. The build React frontend seems to be working fine, tho.