Please make sure your VR headset and PC are connected to same Wi-Fi local network (router). It may also work better on 5GHz line than 2.4GHz Wi-Fi. Some Antivirus software or custom Firewall addons may interfere, and you may want to disable them temporarily. 

Please make sure that you select the stream source in the new browser tab that will automatically open once you add a screen. If you don't see a new browser tab opened, give it a minute and then try to add another screen

To solve this problem, simply set your PC monitor's resolution to a lower value, somewhere around HD (720p). You may need to restart the stream to apply changes in VR (delete the current screen and add a new one)