Choosing a method
There are 2 kinds of ways to hide your Last Seen timestamp, in essence, both are based on the same principle, but differ in the way of execution, so feel free to use whatever suit you best.
Method #1: Automatic – use an app
Method #2: Manual – disconnect
In order to read and write messages without being seen doing so, just manually turn off your smartphone’s Interent and Wifi connection, then open up WhatsApp, read / write messages, close it when you’re done. As soon as you’ll return Internet connectivity, WhatsApp will synchronize with the servers and your messages will be sent without the server getting your last online status update.
iPhone users are in luck, since unlike Android users, you do have a built-in option to turn off Last Seen timestamp, just go into WhatsApp Settings >> Advanced >> Last Seen Timestamp > turn it off.