Unattended access to a Linux host (running Ubuntu)

I configured TV for an unattended access on a machine running Ubuntu 20.04 or 18.04.

The problem is that TV starts only after user's logon in the host machine; therefore it is not possible to perform unattended access. The same problem appears with Ubuntu 18.04 with the same machine. The controlling machine seems to be waiting for some authentication that never arrives. the computer is correctly registered in the the account since immediatly after user's logon ont he machine everithing works perfectly. It seems that the service is loaded but it doesn't work but no error is reported.

Note that the problem appears both configuring teamviewer with or without GUI as well as  on a fresh linux installation and fresh teamviewer host installation (without GUI).

I have no problem with another machine runnig OSX High Sierra on an old(!) IMac 21"where the service is loaded before user logon.

Is there something I'm missing or its just a TV?s flaw?...