The communication between iPhone and Apple TV needs to go over a home network, so Wi-Fi is the very best solution for that.
One could set up a peer-to-peer AirPlay connection, but that is for offline non-DRM content only. And breaks other functionality on connectivity. Not recommended.
One could set up a cellular hotspot on the phone, then the Apple TV could connect to that. You cellular provider might (will) object to such use, especially for extended use. See their terms and conditions, as well as data limits (even “unlimited” plans will have limits).
The communication between iPhone and Apple TV needs to go over a home network, so Wi-Fi is the very best solution for that.
One could set up a peer-to-peer AirPlay connection, but that is for offline non-DRM content only. And breaks other functionality on connectivity. Not recommended.
One could set up a cellular hotspot on the phone, then the Apple TV could connect to that. You cellular provider might (will) object to such use, especially for extended use. See their terms and conditions, as well as data limits (even “unlimited” plans will have limits).