There are many Android and iOS apps that allow you to use WhatsApp Web on your phone.
However, I personally do not recommend that way.
Some apps are paid and others can be used for free with lots of ads included in them.

The safest and most convenient way to use WhatsApp Web on mobile is through a browser.
In this case, you need two devices or a cell phone. One device with the WhatsApp application in it, the other one is used to open WhatsApp Web.
Notes: Regardless of the browser and operating system used, the principle or how it works remains the same. This tutorial is practiced using the Google Chrome browser on an Android phone.
Using WhatsApp Web on Mobile
Here are the steps to use WhatsApp Web on mobile.
- Open the browser on the first phone.
- Visit website.whatsapp.com.
- Tap the menu button (three vertical dots icon) at the top right.
- Tap menu Desktop site. This method will make it look as if you are opening a web page on a desktop device.
- You will be taken to a page that displays a QR code. Keep this page open.
- Open the WhatsApp application on the second phone.
- Tap the menu button (three vertical dots icon) at the top right.
- Choose the menu WhatsApp Web.
- Tap the button DEVICE LINK.
- Scan the QR code by hovering the square in the center at the QR code displayed on the first phone.
Hover the box in the middle (second phone) To scan this QR code (phone first) - Done. WhatsApp Web is ready to use on the first mobile.
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Always Redirected to whatsapp.com
Sometimes, when you open the web.whatsapp.com page on your phone for the second time, you will be redirected to the whatsapp.com page.
The solution, you need to clear the browsing data first.
- Tap the menu button (three vertical dots icon) at the top right.
- Choose the menu History.
- Knock Clear search data.
- Select tab ADVANCED. In section Time spanChoose Last 24 hours then check the box as seen in the image below. If you have, tap the button Clear data.
- Next, you just have to reopen the web.whatsapp.com page.