Can't access express route?

I am working on a mern project and I have this tiny? error: When I try to post on a specific route, express returns 404 and can't find the route I wanted. Everything works fine when I try to post to the index route '/'. Here is a snippet:

in /controllers/imageController

ImageController

exports.image_upload_post = (req,res) => {
   console.log('init file posting')
}

in /controllers/accidentController

AccidentController(works fine)

exports.accident_detail_post = (req,res) => {
   console.log('init form data posting')
}

Here are the routes:

routes/imageUpload route:

const express = require('express')
const router = express.Router()
const image_upload_controller = require('../controllers/ImageUploadController')
router.post('/upload', image_upload_controller.image_upload_post)
module.exports = router

/routes/index route: (the working one)

const express = require('express')
const router = express.Router()
const accident_detail_controller = require('../controllers/AccidentController')
router.post('/', accident_detail_controller.accident_detail_post)
module.exports = router

And finally, server.js:

const express = require('express')
const cors = require('cors')
const mongoose = require('mongoose')
const app = express()
const indexRouter = require('./routes/index')
const imageUploadRouter = require('./routes/imageUpload')
app.use(cors())
app.use(express.json())
app.use('/', indexRouter)
app.use('/upload', imageUploadRouter)

And my question is, why when I try to do something like this:

const submitData = async (e) => {
   e.preventDefault()
   await axios.post('http://localhost:3001/upload', 'test')
}

in the client React app, it returns 404 error? If I try to post to http://localhost:3001/ I get the controller's log 'init file posting'?