Hi @Steve Caparco,
Thank you for posting your question in the Microsoft Q&A forum.
Based on Microsoft document, to join a Cisco Webex meeting from a Teams Rooms device using Direct Guest Join, the Cisco meeting needs to be hosted in Webex Meetings Pro using Cisco Webex web application version WBS 40.7 or later. Therefore, please ensure you meet their requirement. For more information, you can refer to Third-party meetings on Teams Rooms - Microsoft Teams | Microsoft Learn
Based on my research, it looks like Webex and the browser on MTR devices aren't working well together.
Webex may have updated its browser requirements, you may check their list of known issues and workarounds, as a moderator, we do not have enough tools or resources in my environment to diagnose your issue. For further assistance, I suggest you should contact directly Webex Help Center.
You also may check whether MTR devices need additional configuration.
Some MTR devices may require additional configuration to properly join third-party meetings like Webex. You can review the Cisco guide on Configure MTR Device to Join Third Party Meetings - Cisco
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Thank you for your understanding. Have a nice day!
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