Yesterday after I switched from localhost to Heroku hosting, afterwards I got this error: UnhandledPromiseRejectionWarning: MongooseError: Operation user_caches.findOne() buffering timed out after 10000ms

My Web.js File:

app.get('/api/user/:id', async (req, res) => {
    const user = await users.findOne({ discordId: req.params.id }) ?? await users.findOne({ accounts: { $elemMatch: { id: Number(req.params.id) }}});
    const blacklist = await blacklists.findOne({ discordId: req.params.id }) ?? await blacklists.findOne({ accounts: { $elemMatch: { id: Number(req.params.id) }}});

    if (blacklist) return res.send(`No acquirable data, this user is blacklisted.`);
    if (!user) return res.send(`${req.params.id} is not located in the database.`);
    return res.send({ 'discordId': user.discordId, 'accounts': user.accounts });
});

My Schema:

const { Schema, model } = require('mongoose');
const user = Schema({
    discordId: String,
    accounts: Array,
    maxWhitelists: { type: Number, default: 1 }
});
module.exports = model('user_cache', user);

Any help would be appreciated!