If you are on a local network you can use TightVNC https://www.tightvnc.com/. That is the best one I have found. Install the server on the remote clients and just use the viewer from your PC. If you need to connect to a remote network you can use TeamViewer free https://www.teamviewer.com/en-us/ or Google Chrome Remote Desktop https://remotedesktop.google.com/. Just be careful if you are using a work computer as some companies do not allow RDP/VNC software. You can increase the security by not installing the server/service on your primary PC.

what I found out now though is: TightVNC obviously works only between boxes conneced to each other by cable-LAN. Once a box connected with WLAN is involved, it does not work properly. One can open the connection, but very soon the connection seems to get intererrupted, the cursor does not move any more, and then it says that the connection failed.
In my case, it does not matter, because I need it for boxes which are connected via cable