There are differences in how the Webex Teams app connects to cloud-registered and on-premises devices. When connecting to an on-premises device, the content that is shared between Webex Teams app and the room or desk device is always encrypted. However, we don’t enforce certificate verification when an HTTPS session is established with an on-premises device. Verifying certificates would prevent any guest devices working and would be complex to deploy and maintain.
From the customer view in https://admin.webex.com, go to Devices and click Settings. Under Device Discovery, select Allow the Webex Teams app to connect to on-premises registered devices.