SendGrid Reseller accounts do not have access to the updated email Activity Feed, or its "Advanced Search" options for adding additional search filters. Reseller accounts only have access to the legacy Activity Feed.
Activity Feed alternative
Resellers looking for similar data from the updated Activity Feed and Advanced Search options can use Event Webhooks as a substitute for the Advanced Activity Search. SendGrid's Event Webhook will notify a URL of your choice via HTTP POST with information about events that occur as SendGrid processes your email. It is basically a more robust activity feed that posts the events of an email in real-time. For full details, see the Event Webhook Reference.
Please note, the Event Webhook is not retroactive, so you won't be able to search for events that occurred prior to enabling the Event Webhook feature.