Teams Room - No Join Button on MTR device (meetings from external)

This could be a few things. In Skype rooms and since you are on prem what used to happen a lot was TNEF was being stripped from the invite and the join button could not be populated. On your desktop Outlook client, is your reminder pop up having a join button on it or is that gone as well?

Here is a link to a TNEF article to help if this is the issue.

https://community.smarttech.com/s/article/Your-SMART-Room-System-says-Where-s-my-join-button?language=en_US

If you have premiere support, they can look into the properties of the invite and the logs and see why this is happening as well. So probably your best path to resolve the issue would be to open a ticket as if it is not TNEF we could go down more troubleshooting steps, but it might be easier for support to just work with the invite itself to look at the properties and see what is missing.

I have also had luck with making sure to not delete the body from outside invitations as well.