Hello Swampdoc,
First off, please know that not all apps are able to perform over cellular data and not Wi-Fi. Some will require that Wi-Fi connection. For those that are able to use either one, simply turning off Wi-Fi on your iPhone may be enough to make that work. This article explains how to turn that off temporarily: Use Bluetooth and Wi-Fi in Control Center.
You may also need to turn cellular on for certain apps on your iPhone if it is not already on. Learn how to do that here: View or change cellular data settings on iPhone.
Airplay can also be used without a Wi-Fi connection for certain devices and circumstances. Learn more about how to set that up here: Use AirPlay with Apple devices.
We hope this helps. Let us know if we can assist with anything else at all. Take care.