Why doesn't the app indicate 12V battery level?

I will try one last time.

Your secondary 12v battery was not isolated from the car's 12v battery because you had the cut-out voltage set higher the than the cut-in voltage. The battery-isolator did not work properly to protect the car's 12v battery.

The car's 12v battery was drained down so many times that the battery was ruined and will no longer hold a charge. No amount of charging is going to restore the energy capacity in that battery. Why can't you understand that.

If you don't believe me, you can do an experiment to prove that the battery is not good. Remove the secondary battery. Install the Bluetooth battery monitor on the car's 12v battery. Disconnect the negative cable from the battery. Fully charge the battery, then remove the charger. Let the surface charge of the battery come down for four hours. Note the battery voltage at that point. Then, monitor the battery voltage over the next three days. The battery voltage for a good battery should not drop more than 0.2 volts. I think that you will find that your battery drops much more than 0.2 volts. Are you willing to try this?