By Going Offline
Swipe down from your phone. A menu should show up with settings such as brightness, mobile data, and WiFi.
This may not be the case for all phones; you may have to go to the settings app.
Turn off WiFi. If your mobile data automatically activates when there's no WiFi, turn that off as well.
Reply to the message. You will notice that the message will not send. Instead, there will be a clock symbol next to the message. This means that there is no WiFi, and thus, the message hasn't been sent.
Turn on WiFi. The text will now send, and either a single or double check will show up, signifying that the message has been sent and read, respectively. This message will send, without showing that you were online at the time.
By Replying from Push Notifications
Wait for a notification. If one doesn't show up, you may have to activate notifications for WhatsApp by going to the settings app.
Click on the "Reply" button. When you get a notification, if you drag down, there should be a "Reply" option. This may differ for some phones, but there should be a similar option.
Reply to the message. Don't click on the notification. Instead, input your reply, and then click on "Send".
Your message is now sent. It won't say that you are online, but if you click "View" instead, you'll be taken to the app, and it will show your online status.
N.B.: Turn your mobile data connect during offline text send.