- Set notification preview to never
- Enable DND
- Remove apps and disable notifications that don't respect the above settings.
For some testing you might be able to disable cellular and then block the APNS servers or all internet or just Apple's servers, but local notifications will still trigger since they are designed to do so.
Your best bet for controlling a demo is to control all the apps and settings closely rather than using a fully loaded device with many sources of distraction or interruption.
Source. It looks like you need to make sure DND is set to
Change Your Settings
If you don't want to be disturbed at a certain time, you can set a schedule. Tap
Settings > Do Not Disturb and turn on
Scheduled. Then set a time.
You can also choose when you want to receive alerts, calls, and notifications:
- Allow Calls From: Allow calls from everyone, no one, your favorites, or specific contact groups stored on your device or iCloud.
- Repeated Calls: If someone calls you twice within three minutes, the call won't be silenced.
- Silence: Choose to silence calls and notifications always or only when the device is locked.