If your phone gets missing, recovering your WhatsApp account takes completely different steps. In this case, you do not have access to your sim card and so cannot just log in.
The first thing you should do is to inform everyone including WhatsApp. If people are aware that you no longer have control over your account, they will be prepared for any fraudulent chats.
Now, you should send a mail to Whatsapp at email@example.com. You can state the details of the incident to the mail, but make sure you add “Lost/Stolen: Please deactivate my account” in the subject and text body and also provide the mobile number registered with the WhatsApp account.
Please ensure that you are sending this mail from the email address registered in your two-factor authentication process. If you did not do the two-factor authentication before your phone got lost or was stolen, then you can use any of your email addresses.
Once the account has been successfully deactivated, the thieves will no longer be able to use it to chat with your contacts. Now you have 30 days to recover your account if you still want to be able to access all your saved chats and media backed up in the cloud. It you delay this beyond 30 days, you will lose access to all that data.