Diagnosis
The double dash means the main iSmartGate Pro unit has completely lost communication with the tilt sensor. It's not broken yet, but it's not talking.
How to Fix It
Start with a simple reset of the sensor's connection. Take the sensor down and bring it right next to the main iSmartGate unit. Open the battery cover, remove the battery for 10 seconds, and put it back in. You should see a small LED on the sensor blink. Then, in the app, go to Sensors > Tilt Sensor > Advanced and look for 'Refresh' or 'Reconnect'. Tap it. The app should search and re-establish the link, and the angle should reappear.
If That Didn't Work
There might be a 'zombie' sensor conflict. In the app, go to that same Advanced menu and look for 'Remove Sensor' or 'Forget'. Do that. Then, with the sensor still next to the main unit, go to 'Add Sensor' in the app and follow the pairing process from scratch, which usually involves holding a button on the sensor.
Heads Up
After you get it working next to the main unit, re-mount it on the door slowly. If the '--' comes back as you move it to its final spot, you've got a signal dead zone. You might need to relocate the main iSmartGate unit to a more central spot in the garage to get good coverage.