Apple TV HD, tvOS 12
That unit hasn’t seen software updates in years. We’re on tvOS 16.2 now. It is recommended to keep all your devices up-to-date for best support, functionality, general performance, and stability. When you get remote functionality, you will want to update the system software. Update your Apple TV - Apple Support.
No remote is set up on my iPhone or iPad
The software remote in Control Center is standard installed on any iPhone or iPad with iOS 12 or later. Setting up only has two actions: selecting the Apple TV unit to connect to, and (one time) confirming by copying the pairing code, if the mobile device wasn’t paired yet.
If you lost the remote for your Apple TV - Apple Support
Buy a Siri Remote (3rd generation) - Apple Store online
(You may buy earlier models remote from third party resellers that still have them.)
For pairing a replacement remote, follow the pairing procedure. Pair the Siri Remote with Apple TV - Apple Support. There are minimum system requirements for certain models remote though:
Identify your Apple TV remote - Apple Support
- The (black) Siri Remote (1st generation) works with any Apple TV (4th generation or later), i.e. tvOS 9.0 or later.
- Note that the Siri Remote (2nd generation) requires tvOS14.5 or later.
- ➔ Note that the Siri Remote (3rd generation) [what is sold as new now] requires tvOS 15.4 or later. I.e. you need to update first.
Update your Apple TV - Apple Support
To update, you will need a functional remote; the software remote or a physical remote. Even the old style Apple Remote (aluminum) will do. Borrow or buy a Siri Remote (1st generation), or ask Apple for assistance with updating (you may need to bring in the Apple TV box).