We love Teamviewer and have used it for many years.
We are so committed to it that we purchase the commercial licence, at quite some expense, some time ago.
However there is a problem.
Each year Teamviewer release a new version of their software and require us to buy a new licence at many thousands of dollars. We originally bought the licence so we did not have ongoing expenses and could therefore keep our support costs to our customers down.
Most small businesses get the latest Teamviewer on their PC as the free version and it automatically updates to the newest version. Then when we try to initiate support we cannot because our paid version is incompatible with the free version.
The free version runs at the startup of your PC and it would allow anyone with the codes to access your PC. This includes the people who built your PC, the IT company that provided you support, and the installation company that installed or fixed your software online. We don't consider it safe.
Our version is specifically keyed to our purchased version and we deactivate it once our service is performed.
We use Teamviewer version 9.
It is available here:
https://community.teamviewer.com/t5/Knowledge-Base/Download-TeamViewer-8-and-9/ta-p/78348