You can take an old Android tablet and use it as a monitor, and it's surprisingly easy.

spacedesk is a free app that lets you use an Android tablet as a second display for your Windows PC. Learn how to set up and use it.

I also have a S9 ultra tablet that is wireless to the same router. Can I use my S9 tablet as a second monitor? Or does my desktop have to be connected wirelessly also for this to work? Thanks!