I have the BT smart baby monitor using the smart controls app.
It works fine on Wi-Fi. However, it’s never worked when I’m on 4G. I get an error message that says “something went wrong on our end. We are working diligently to fix this so please try again in a few minutes”.
I have mobile data access turned on for the app in my settings.
If your product is within warranty:
By phone: 0808 100 6554
Monday to Friday 9.00am to 5.30pm
By email: Click here for email support
If your product is out of warranty contact BT's recommended agent
By phone: 0800 980 8999
It seems more likely that you can only view it while connected to your own network, and not if you try to connect from an external Internet connection.