My Apple TV won’t connect to airplay on my I phone.

I recently purchased a second hand Apple TV 3rd gen, I went through the set up process as normal and most features work fine. However the airplay feature will not connect from my phone to the Apple TV, it won’t rven discover it. I downloaded the Apple TV remote app and that works and finds my Apple TV fine. I did phone tech support and they said it could be due to my I phone being in the latest iOS and the Apple TV being an older model it will

Stop the connectivity. But I’m not too sure on this as I can still connect using the remote app. Has anyone got any any advice on this? Thank you.