Socket io is not working after deployment(MERN app)

sorry, it might be a simple question, and i did researched all day along.. but still couldn't find a solution..

I deployed MERN app via Netlify and Heroku, and everything is working except Socket.io.

My code is like below..

for server side,

   const app = express()
    var httpsOptions = {
        key: fs.readFileSync("./test-key.pem"),
        cert: fs.readFileSync("./test-cert.crt"),
        ca: fs.readFileSync("./test_ca.crt"),
        requestCert:false,
        rejectUnauthorized: false
                    }
    const server = https.createServer(httpsOptions, app)
    var io = socketiO(server)


    io.on("connection",(socket)=>{
    
        socket.on("Join", async({sender})=>{
            console.log("connection accepted")

for client side:

socket = io("https://herokusiteaddress")
socket.emit("connection")
socket.emit("Join", {sender})

And it gives error message:

Get https://netlifyappaddress/socket.io/?EIO=4&transport=polling&t=NePfNTm 404
polling-xhr, js:198

Does anyone know how to fix this..? I am newbie on the deployment.. Thank you for your help!