How to mock external service call inside a rest api in Node.js

I'm trying to test a rest api which calls an external service.

server.js:

const express = require('express');
const app = express();
const router = express.Router();
const redirectUrl = require('../utils/redirection')

let baseUrl = 'myUrl';
let externalUrl = 'externalUrl';

router.get('/redirect', async (req, res) => {

  const { productName } = req.query;

  baseUrl = baseUrl + '/' + productName;
  externalUrl = externalUrl + '/' + productName;
  await redirectUrl(res)(timeOut, externalUrl, baseUrl)

})
app.use(router);

app.listen(3000);

utils/redirection.js:

redirectUrl = res => (timeOut, url, redirectUrl) => {
return new Promise((resolve, reject) => {
    let cleared = false;

    const timer = setTimeout(() => {
      cleared = true;
      return resolve(res.redirect(302, redirectUrl));
    }, timeout);


    return fetch(url)
      .then(
        response => {
          if (!cleared) {
            clearTimeout(timer);
            const {location} = response.headers;
            return resolve(res.redirect(302, location));
          }

          return null;
        })
      .catch(err => {
        if (!cleared) {
          clearTimeout(timer);
          return reject(err);
        }
      });
  });
}

test.js:

const requrest = require('request');
const chai = require('chai');
const server = require('../server/server');
const { expect } = chai;
describe('My test', () => {
  it('should redirects to the suitable page', () => {

    const { status, headers } = request(app).get('/redirect')
    expect(status).to.equal(302);
    expect(headers.location).to.not.equal(0);

  })
})

So, the test executes the api by providing the provided baseUrl and externalUrl which are set in the environment configuration. However, I wan to mock the call of redirectUrl when the route would be tested. I tried to use nock:

const request = require('request');
const chai = require('chai');
const server = require('../server/server');
const { expect } = chai;
const nock = require('nock');
describe('My test', () => {
  it('should redirects to the suitable page', () => {
    nock('fake url')
    .get(`/${productName}`)
    .reply(302, {
        headers: { location: 'url/to/redirect/page'}
    })
    const { status, headers } = request(app).get('/redirect')
    expect(status).to.equal(302);
    expect(headers.location).to.not.equal(0);

  })
})

When executing the test, the nock mock has never been called. Any suggestion to mock the function call of redirectUrl inside a rest api?