This sounds like the ISP's DNS may be flaky or out of date.
Next time you have the PC change the DNS to a public DNS like 8.8.8.8
I don't write at length about public DNS options or how to change as there are many fine articles on the web along with why we do this.
Hi,
I am trying to help out an elderly friend with his laptop.
I tried to talk him through installing TeamViewer so I could assist remotely.
He got an error an error saying secure connection failed, and he was unable to download the software.
I collected his laptop, brought it home, and installed TeamViewer with no problems.
It also loaded and ran correctly, displaying the ID and password needed for me to remotely access it.
When he had the laptop back, he loaded TeamViewer, but the ID and password wasn't connected, and he got an error message saying may be using a proxy server.
I have just tried to get him to access AnyDesk.com, and once again, he gets the secure connection failed message.
His internet is working, he can access his emails on the gmail web portal, so I am at a loss to understand what is going on.
Can anyone shed any light?
Robert.

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