How can I keep a BLE app alive all the time?

Question

I'm trying to connect an App to a custom system with BLE that I added to my dog's door, which allows me to open/close it remotely as well as to know when is open "illegally" so it reproduce an alarm in my phone.

I've added the BLE class to the Main Activity and manage it from there, using fragments to see the data and sending commands.

But the problem I'm having is that, after some time, the App just close itself, breaking the BLE connection so I can't receive updates in "real time".

I don't know if is possible that the system don't kill the App, I've hear about services and that they keep the connection alive, but I'm not sure if they respect the other parts of the App or if the App get killed but the service don't. I've never worked with services before.

I've added the REQUEST_IGNORE_BATTERY_OPTIMIZATIONS permission and also disabled the memory optimization as well but it still get killed.

When the app is killed and I press the App icon to open it, it looks like a fresh start and it should be keep alive all the time. Otherwise I will have "security issues" if something happens while I'm not connected.