Normal WiFi/USB/Ethernet connection is like this:
App <–internet–> Server <–internet–> Hardware
But with Bluetooth/BLE it is a direct, short range and limited App-to-Hardware link, but still requires the App-to-server link to function:
Hardware <–BT/BLE–> App <–internet–> Server
If you don’t want any internet then you still need a standalone server for the app to connect to:
Hardware <–BT/BLE (or WiFi via LAN)–> App <–local area network connection–> Local Server
If you first connect the app to the hardware via BT, and then disconnect the app device (phone tablet) from the internet, the app will keep working working, via BT to the hardware… but as soon as you leave or minimise the app, it seems to need the internet (or connection to the server) to reconnect again.
So simple speaking, the App needs a Server, one way or another.