Computers Appearing Offline

Good Morning,

I am hoping someone can help! I am running teamviewer 12 and have around 300 clients connected via unattended access. I have been extremeley impressed with the service so far! However as of a couple of weeks ago I am having saved previously working clients, showing online then offiline. Even when they are online I am unable to connect however. On top of this a large chunk of my clients are showing offline completely. Now nothing on my network has changed no changes in AV or Firewall on these particular clients either. I am noticing on these problem machines I am getting the "please check your internet connection, most likely you use a proxy server etc..."

Has anyone come across this message before?! Is it a simple fix, this is now causing huge issues as I am unable to remotely connect to these machines.

Please help!

Kind Regards,

Nick