Since you have already contacted them and have order details and a video, you can lodge a formal consumer complaint to the National Consumer Helpline (NCH) with your previous complaint reference numbers and all order details, including the missing item and the video evidence.
Before this, you may also escalate the case to Grievance Officer of Zomato by following these instructions:
To lodge a consumer complaint with NCH, you may call at the helpline number 1800-11-4000 / 1915 or use these steps for raising your case online: