As per WhatsApp Guidelines, the chat session expires after 24 hours of your customer's last message sent time. This is a rule of WhatsApp APIs and WATI does not have control over it.
After the 24 hours expiry, you can only send 'Template Messages'
If your customers reply to the template, the session is restarted and the expiry time would be increased.
We recommend submitting a template such as below where you can let your customers know that you want to share an update with them.
Upon clicking the 'Tell me more' a reply is initiated from the customer's end to the business and the session is restarted.
We hope this helps!