All Android Firewall apps BLOCK ALL Outgoing data automatically when the app is enabled as a default action. The Firewall DOES NOT disrupt or block any incoming data into your device. ONLY outgoing data are Blocked. Basically, no data leaves your device when the Firewall is enabled; the Firewall app then informs you what app tried to connect outbound. It is then up to you if you will ALLOW such a connection the next time, OR choose to RESET the connection then ALLOW. OR BLOCK the outbound data connection wherein the DATA sent by the app will be dropped by the VPN sinkhole (for NoRoot Firewall apps), thus fooling the app who attempted for an outbound connection that the data was "sent".
You can use Airplane Mode to Block All IN/OUT connections, and isolate your device from all radio traffic. Though each type of phone or device, brand, version of Android, have different ways to achieve True Airplane Mode action. You can enable Airplane Mode, take out batteries (believe you me) usually gives you true isolation. But when the battery is integrated on the device then it's a bit tricky; You should then first go to SETTINGS/WIRELESS NETWORKS/SIM CARDS and Deactivate the SIM Card or Cards if there are two; THEN proceed to Enable Airplane Mode for better isolation.
*you can try to scan your area with your device using another android device by using Bluetooth Firewall app and check. In my case, my Lenovo Tab3 730 was picked up by an Android phone running BTFW app even when Airplane Mode was Enabled !!!
So there has to be a TRUE Airplane Mode state for each and every Android device.
- You can download compatible apps that will lockdown WiFi, and Bluetooth to secure any attempt by the Android device to try to connect by other ways because in the Android system, when you disable Mobile Data Connection, WiFi /Bluetooth will AutoStart.
Such apps include Device Control by Expert Systems Co. and D-Vasive app by D-Vasive Inc. In my opinion, these apps WITH the Airplane Mode coupled as above, might be overkill, but I presented the answer, hopefully on the level of your concern reading your question, if not the size of the TinFoilHats we are wearing? 😁