Can I use an Apple TV to present wirelessly without being connected to a WiFi network?

Yes, to present wirelessly using an Apple TV without being connected to a WiFi network you would use Peer-to-Peer AirPlay. Peer-to-Peer AirPlay uses the WiFi and Bluetooth on the device, but does not require to be connected to a WiFi network or the internet.

Here is a link to an Apple support document that explains how to set it up: https://support.apple.com/en-us/HT204289