I believe you can signup on WhatsApp web. You simply need a phone number to verify your account.
The link follows to https://faq.whatsapp.com/faq/web/28080002
So I admit, I'm also struggling to see what the benefit of using WhatsApp through Browserling is.
Insert a 'free' sim card. Activate it. Write down the phone number.
Actions on the tablet:
Enable installation of unknown sources.
Download the apk file at https://www.whatsapp.com/android/
Execute the apk file
Use the phone number to activate Whatsapp