Huion Drawing Tablet pen not working in External Monitor, this is because by default it takes main screen, to change or if you want to use pen in external or other monitor in that case go to Huion App setting and inside there select displays and now pick the screen where you want to use the Pen if you want to use tablet on both screen select both screen but it also reduces the working area in your drawing tablet. I will recommend you to use on on screen only for better work flow.