Sessions not working on the site (MERN and heroku)?

Question

Heroku

Hello!
I set up cors and deployed the MERN app on Heroku. For some reason the sessions are not working. Moreover, if you send requests from postman or insomnia, then everything works. Also, it works locally too. How to fix sessions?

const express = require('express');
const mongoose = require('mongoose');
const cors = require('cors');
const bodyParser = require('body-parser');
const cookieParser = require("cookie-parser");
const passport = require('passport');
const LocalStrategy = require('passport-local').Strategy;
const User = require('./models/User.js');
const session = require('cookie-session');
const dotenv = require('dotenv').config();

// Routes
const userRouter = require('./routes/user-router');
const bookRouter = require('./routes/book-router');

// const corsOptions = {
//   origin: 'http://localhost:3000',
//   credentials: true,
//   optionsSuccessStatus: 200
// }

passport.use(new LocalStrategy(
  function (username, password, cb) {
    console.log(username, password)
    User.findOne({username}, function (err, user) {
      if (err) {
        return cb(err);
      }
      if (!user) {
        console.log('Invalid username')
        return cb(null, false);
      }
      if (user.password != password) {
        console.log(user.password);
        console.log(password);
        console.log('Invalid password');
        return cb(null, false);
      }
      return cb(null, user);
    });
  }));


passport.serializeUser(function (user, done) {
  done(null, user.userId);
  console.log('Serialize')
});

passport.deserializeUser(function (userId, done) {
  User.findOne({userId}, function (err, user) {
    if (err) {
      return done(err);
    }
    console.log(user);
    console.log('Deserialize')
    done(null, user);
  });
});

const app = express();
app.use(cors({
  origin: 'https://still-waters-66948.herokuapp.com',
  credentials: true
}));
app.use(cookieParser('keyboard cat'));
app.use(express.json());
app.use(express.urlencoded({extended: false}));
app.use(session({secret: 'keyboard cat', resave: false, saveUninitialized: false}));
app.use(passport.initialize());
app.use(passport.session());

app.use(bodyParser.urlencoded({extended: false}));

app.use(userRouter);
app.use(bookRouter);
mongoose.connect(process.env.DBURL, {useNewUrlParser: true, useUnifiedTopology: true});

const db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function () {
  console.log('DB connected');
});

app.listen(process.env.PORT, () => {
  console.log('Server has been started on port ' + process.env.PORT);
})

Server code