Hmm. That's definitely odd that a sleep setting would not work. Try finding an app that forces the screen to stay on. Like an app that intentionally runs in the background and keeps the tablet on. That might be the quickest way to solve your problem. Something like this:
Note: I have never used this app and I do not know if it works. I am not endorsing this app in any way, it's just something I found that may help you out.
If you do not have anything important on the tablet, you can always try resetting to factory defaults and seeing if your sleep settings will work after that. Good luck.