Honestly: poor. Once the festival is running there is practically no signal.
The site plan still helps: “Where am I?” puts you on the plan by GPS, which needs no network. Under “My places” you can save your tent or a meeting point, and the plan later shows you the distance and direction to it. A place can be shared as a grid square plus a link – at a pinch you can read that out or send it by text.
Also install our schedule as an app before you set off; it works offline. And agree on fixed meeting points, it saves a lot of searching.