Able to connect - then offline after I login to Windows

Really strange problem. Windows 10 Pro 1709 to Windows 10 Pro 1709. Both computers have free version. Both are up-to-date on version 13. When I login to the remote computer via unattended mode - I get a Windows logon window. Everything is fine. But after I login to Windows, Teamviewer goes offline. Only works again when PC is rebooted. Why? Host is on wifi, remote machine (mine) is hardwired. WiFi is stable since it works fine for a long time at windows logon prompt (no lagging on mouse or keyboard strokes) through teamviewer. Any ideas anyone?