New Apple TV

I have nearly 1,500 movies and a couple of thousand tv episodes

But are those iTunes Store purchases? Only those would show in the TV app.

Are those purchases bought using the same Apple ID account as now active on the Apple TV box? It wouldn’t show purchases from a different account.

For Home Sharing content, use the Computers app instead.

Then that should show the same library.

You may want to contact Apple Support about that.