Devices appearing offline

QT

We have ~300 hosts running team viewer. About 2 weeks avgo, over half of them started showing up with question marks next to them in the Team Viewer Console and we were unable to connect to them. Now the question marks are gone, but they appear offline. We have tried rebooting the hosts that appear offline. It does not seem to help. These hosts are deployed at 20 remote sites across the country, and at each site, about half of the hosts are not working any more. any ideas what could be wrong? We are running veriosn 6.0.17222