Hi @Rib,
Thank you for posting to Microsoft Community.
Based on your description, I understand that the Cloud only Microsoft Teams Rooms cannot be added to room list synced from on-prem to the cloud.
I’m afraid that it is common to happen since the Microsoft Teams Room are cloud only.
You could refer to the deployment guide of Microsoft Teams Rooms - Microsoft Teams | Microsoft Learn for more information.
If you have anything else need help, please let me know.