Mongodb – Intermittent Unhandled Promise Rejection – MongoServerSelectionError: connection timed out

I am getting below error randomly.
Here is my stack

• App deployed on vercel is talking to Atlas mongodb (free tier)
• Nodejs – 18.16.1
• nextjs 13
• mongodb package version – 5.8.1

ERROR   Unhandled Promise Rejection     {"errorType":"Runtime.UnhandledPromiseRejection","errorMessage":"MongoServerSelectionError: connection timed out","reason":{"errorType":"MongoServerSelectionError","errorMessage":"connection timed out","reason":{"type":"ReplicaSetNoPrimary","servers":{},"stale":false,"compatible":true,"heartbeatFrequencyMS":10000,"localThresholdMS":15,"setName":"Cluster0-shard-0","maxElectionId":null,"maxSetVersion":null,"commonWireVersion":0,"logicalSessionTimeoutMinutes":null},"stack":["MongoServerSelectionError: connection timed out","    at Timeout._onTimeout (/var/task/node_modules/mongodb/lib/sdam/topology.js:278:38)","    at listOnTimeout (node:internal/timers:569:17)","    at process.processTimers (node:internal/timers:512:7)"]},"promise":{},"stack":["Runtime.UnhandledPromiseRejection: MongoServerSelectionError: connection timed out","    at process.<anonymous> (file:///var/runtime/index.mjs:1186:17)","    at process.emit (node:events:525:35)","    at emit (node:internal/process/promises:149:20)","    at processPromiseRejections (node:internal/process/promises:283:27)","    at process.processTicksAndRejections (node:internal/process/task_queues:96:32)"]}
[ERROR] [1692852863980] LAMBDA_RUNTIME Failed to post handler success response. Http response code: 400.
RequestId: 971d9da3-a51d-4511-a561-22d5702be848 Error: Runtime exited with error: exit status 128

When this error occurs, server responds with http 500 error code.

Can someone tell me why this would happen? Is it due to free tier of Atlas Mongodb?

Below code shows how I am connecting to mongodb.
Clientpromise is being imported and then used to do db operations in other files.

import { MongoClient } from "mongodb";

//mongodb+srv://sjhfjhfjhf/ongodb.net/awesomedb?serverSelectionTimeoutMS=60000
const uri = process.env.MONGODB_URI;
const options = {
  //useUnifiedTopology: true,
  useNewUrlParser: true,
  serverSelectionTimeoutMS: 59000,
};

let client;
let clientPromise;

if (!process.env.MONGODB_URI) {
  throw new Error("Please add your Mongo URI to .env.local");
}

if (process.env.NODE_ENV === "development") {
  // In development mode, use a global variable so that the value
  // is preserved across module reloads caused by HMR (Hot Module Replacement).
  if (!global._mongoClientPromise) {
    client = new MongoClient(uri, options);
    global._mongoClientPromise = client.connect();
  }
  clientPromise = global._mongoClientPromise;
} else {
  // In production mode, it's best to not use a global variable.
  client = new MongoClient(uri, options);
  clientPromise = client.connect();
}

// Export a module-scoped MongoClient promise. By doing this in a
// separate module, the client can be shared across functions.
export default clientPromise;

Here is how I am firing the query

import { ObjectId } from 'mongodb';
import clientPromise from '../../lib/mongodb'
import { connectToDatabase, slugify } from './blog';

export async function saveAdToMongoDB(adObj) {
  try {
    const collection = (await clientPromise).db().collection("ads");  
    let catslug = slugify(adObj.businessCategory)
    let slug = slugify( adObj.businessName + "-" + adObj.city + "-" + catslug)
    adObj.slug = slug ;
    adObj.catslug = catslug;
    let advertisementObject = {...adObj, status:"pending",image:""}
    let result = await collection.insertOne(advertisementObject);
  } catch (err) {
    console.error("Failed to save ad to MongoDB:", err);
  } 
}

I upgraded mongodb and nextjs packages but it did not help. Most of other solutions on internet are talking about firewall and ip whitelisting. But I do not think that’s causing the issue.