Upgrade MacOS via TeamViewer ?

I am connecting to a display-less Mac Mini via TeamViewer. I started the upgrade of MacOS to Sierra, but it seems that TeamViewer no longer functions. After waiting for 30 minutes after the reboot of the Mac, it is still not visible from TeamViewer.

I can ping the rebooted Mac, but it is not accessible frm TeamViewer anymore.

Am I stuck ? Will I have to go and get a keyboard, mouse and monitor connected to the Mac ?

Very best regards,