There is a nifty application called SpaceDesk for Android, IOS and Windows OS that makes it possible for you to run it and use your old tablet as a second monitor and it takes no more than a minute of your time to set it all up and configure.
I will show you a very easy video guide on how to use your Android Phone, Tablet or IOS iPhone or iPad device as a second monitor display for your windows laptop or desktop computer with an app head on to SpaceDesk.Net and you can find it on Google Play Store for your Android devices.
You can even use a second Windows laptop or second desktop computer as another display just install the same on your other Windows PC and use it as a second monitor through internet or wireless.
Disclaimer you need to be connected to the same network for the setup to work. In this video I used an old Samsung Tablet from 2014 with android version 5 installed on it and it worked like a charm so you can use an old tablet and install Spacedesk on it like in the video.