This article will help you troubleshoot getting a device that is locked to an app back online
When your Supervised device that contains an App Lock profile has lost it’s WiFi signal and someone clicks a link, the app will get stuck trying to launch that page but won’t ever complete. The device will stay in this state even if the device becomes connected to WiFi again. If this happens you’ll need to manually reboot the device and it will launch your app again.
If this still doesn’t correct the issues you’ll need to connect the device to your Mac and open Apple Configurator to get WiFi access back to the device. Once this is done then you can remove the App Lock profile from Manager, connect the device to the proper network and then place the tablet back in App Lock mode.