I've got 2 rooms that up until yesterday worked fine. They each use an account tied to the conference room they are in. ex. firstname.lastname@example.org. The accounts work fine anywhere else you use them BESIDES the Microsoft Teams room GUI. I even plugged in my UPN into the MTR GUI and it logged right in.
I logged into the Windows desktop on the local NUCs that run the MTR and logged into the same room accounts via the Teams client and the OWA no issues.
I've contacted support but they were less than helpful and have me sending screen shots and UPNs... as well as run powershell commands that do not work on any PShell platform I have.