Node/Express req.session.user "undefined"

Question

I am a beginner Node.js developer and am working on a web app for which I need session management. I'm defining Express session like the following:

app.use(cookieParser());
app.use(session({
    secret: 'secret',
    resave: false,
    saveUninitialized: true,
    cookie: {
        expires: 600000
    }
}));

And set the session variable in /signup (not shown) and /login. However, if I try to access req.session in any other route, it shows up as "undefined". Does anyone have any input on this?

router.post('/login', async function(req, res) {

    console.log(req.body);
    var email = req.body.email,
        password = req.body.password;

    let user = await User.findOne({ email: email });
    if (!user) {
        res.status(400).send("Failure");
    } else if (!bcrypt.compareSync(req.body.password, user.password)) {
        res.status(400).send("Failure");
    } else {
        req.session.user = user;
        res.status(201).send("Success");
    }
});