Set up the Apple TV Remote on your iPhone, iPad, or iPod touch - Apple Support
How old? The software remote in Control Center on your iPhone is compatible with Apple TV (3rd generation or later), not 2nd generation or earlier. What is your model Apple TV? Identify your Apple TV model - Apple Support
You may need a working physical remote to help set up the network connection if needed, so that both the iPhone and Apple TV box are on the same network.
You may need a working physical remote to help set up the remote for Apple TV (3rd generation), as a pairing code will have to be entered on Apple TV.
A temporarily borrowed physical remote would help during setup of the software remote.
You can buy a proper replacement remote for optimal use.
Buy an Apple Remote (aluminum) for use with Apple TV (3rd generation or earlier).
Buy a Siri Remote for use with Apple TV (4th generation or later).
Replacement remotes will need to be paired or linked: If your Apple TV remote isn't working - Apple Support