Ran some tests at my cuz house to use his S5.
Appletv mirroring can work with no internet/no wifi network as long as you have both wifi and blutooth turned on your device. On the apple tv itself, airplay should be on by default. I think Bluetooth is on by default too, can't remember.
But yeah on your iphone, you don't actually have to be connected to a wifi network for it to let you mirror to your appletv.
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The next test was with a samsung galaxy 5. The free version of the allcast app lets you show your music, gallery photos, vids, and video app files. It also lets you show your google plus, google drive and dropbox stuff. You can also use it for DNLA server with it's media server option (good if you use kodi, plex, nas, etc)
It does not mirror though. I wish they had a mega.co.nz option.