Function doc.save() is not a function

I want to save doc to DataBase but when i create new User and try to save it function .save() not working
I dont have any variants to solve this problem. I tried to do everything what i found in Internet
I check some overflow posts but anyone helps me

My Index.js file:

const app = express();
const port = 5000
app.use(express.json());
app.set('view engine', 'ejs');


const uri = '<my link>';
// Create a MongoClient with a MongoClientOptions object to set the Stable API version
const client = new MongoClient(uri,  {
        serverApi: {
            version: ServerApiVersion.v1,
            strict: true,
            deprecationErrors: true,
        }
    }
);
client
    .connect()
    .then((res)=>console.log('Connected'))
    .catch((error)=>console.log(error));

app.post('/auth/register',async (req, res)=> {
    try {
        const user = new User(req.body);
        await user.save();

        res.json({
            success: true,
        });
    } catch (err) {
        res.status(500)
    }
});
app.listen(port, () => {
    console.log(`Example app listening on port ${port}`)
})

My model/user.js file

import mongoose from "mongoose";

const UserSchema = new mongoose.Schema({
    name: {
        type:String,
        required: true
    },
    email: {
        type:String,
        required: true,
        index: {unique: true}
    },
    passwordHash: {
        type:String,
        required: true
    },
    access: {
        type: Boolean,
        default: 0
    },
},
    {
        timestamps: true
    });

const User = mongoose.model('User', UserSchema);
export default User