So I think I found a solution to the same issue after lots of searching.
It is related to an article I saw here (https://www.teamviewer.com/en/help/1057-how-do-i-use-teamviewer-on-headless-systems)
The problem is that when there is NO monitor connected then for TeamViewer:
A.) Content which is related to hardware acceleration (i.e. browser content or 3D applications/games) can not be displayed properly
B.) The whole user interface could not be displayed (black screen)
So after I read that I left a monitor connected and used TeamViewer from my phone. Everything worked fine including Wattman BUT when I disconnected my monitor I just saw a transparent window and nothing else (related to the first problem mentioned above).
Thus the solution is to either leave a monitor connected to the rig or use an HDMI terminator. I did the second and it works fine