If you’re remoting to a computer that has multiple monitors, you can only switch between monitors if you’re using the TeamViewer Full Client. At the time of writing, it doesn’t seem that the web interface supports viewing more than one monitor.
To view other monitors in TeamViewer, remote connect via the Full Client, then click “View” at the top of the screen after you’ve connected and then click on the monitor you want to switch to.
I’ve read that there’s also a way to view multiple monitors in multiple screen connections, but I’ve not worked out how to do this yet. I may do another post on the subject in the same forum, or perhaps I’ll just reply to this one. Watch this space…