How to set up an iPhone or iPad to act as an Apple TV (4K) remote?

I can't find the Apple TV (4K) remote, and tried to set up my iPhone or iPad as the remote . But the Apple TV was on a different Wifi router before, and the iPhone and iPad remote app turned the wheel repeatedly and can't find the Apple TV device. Is there any way to make it work? Such as moving the iPhone or iPad 2 feet away from my Apple TV and have them "hooked up"? I guess one possibility was if that Apple TV sent a token to iCloud for my account long before, and can let me download that token to my remote app to authenticate it is the same user trying to use the remote functionality but I guess such functionality is not there (yet)? Is there any method at all? A side note is, if a new remote needs to be purchased, I supposed it can still hook up to my existing Apple TV (4K), or else there really is no point in selling a remote by itself?