Hey, I think this might help:
1. Download and install the Not Last Seen app from here (root not required).
2. Open the app and enable ‘Block Last Seen!’
3. Open the WhatsApp, now you will see the data connection went off automatically as soon as you open WA.
4. Read and send the messages, then close the WA screen.
5. Once you’re out of the WA screen, the data connection will enable automatically and will send the composed messages.
*Information from andromint