iTunes remote app on iPad

Assuming you mean this iTunes Remote App

https://apps.apple.com/us/app/itunes-remote/id284417350

Still works fine. What issue are you having exactly?

Are you trying to control a Mac or a Windows PC? If a Windows PC, is it running iTunes or the Aple Music App?

The Apple Music app on Windows no longer supports the Remote app unfortunately. But iTunes should still work fine.

click here ➜ Set up the iTunes Remote app for iTunes on PC - Apple Support

So the library does not appear to be selected, or it does appear, but it does not open?

Or it appears and opens, but does not start playing anything when you select a song and hit play?

Does it issue any type of error? If so, what does it say?

You can always try to unpair the remote from the Music app on your Mac, and then pair it again.

On your Mac, go to the Music App ➜ Music Menu ➜ Settings ➜ Advanced ➜ Remotes ➜ Forget all remotes.

Then on the iPad, tap on the Libraries button at the top left corner on the "<More" text until you see the Libraries button, then on Settings and then on Add a media library. A code will appear on screen.

Go back to the Mac, open the Music App and an "iPad" entry should appear under the Devices section of the side bar with a play button icon next to it. It won't be the one with your iPad's name though. Click on it and it should show a screen asking for the code. Enter the code shown on your iPad to pair it.

click here ➜ Set up the iTunes Remote app for Music on Mac - Apple Support

Yeah, that's it. I use it to connect with my music library on my iMac from my iPad if I'm in another room and I'm too lazy to walk over to the computer. It suddenly stopped being able to access the library a couple of weeks ago.