To activate the Online (LastSeen) tracking of a number in the Online Monitor app:
  1. You must open this page from a computer or a device different from the one you have WhatsApp installed on so you can scan the QR code.
  2. Make sure you have received at least one message from the number you want to monitor in the last 7 days. If not, send them a random message and wait for a reply. (THEY MUST REPLY TO YOUR MESSAGE)
  3. Link the WhatsApp used to exchange the message in step 1 with the Online Monitor app, by scanning the QR Code from this page. (Steps 3, 4 and 5)
  4. Open WhatsApp on your phone
  5. Tap Settings
    → Linked Devices → Link a Device
  6. Point your phone at the QR code to scan it
  7. Now go back to the Online Monitor app and wait for the status of the number to be tracked to change from "Starting monitor..." to "Monitoring✅".
  8. (Optional step) Now you can remove the Online Monitor linked device from your WhatsApp (you will have to repeat all the steps every ~15 days). Or just leave it connected for automatic updates.