To use an opening sensor you need to make sure you have a 2nd generation 1Control LINK device and it needs to be within approximately 15 metres of your sensor.
In the case of a door opening sensor, make sure it is powered: remove the plastic tab from the battery compartment. 
In the case of a gate or garage opening sensor, make sure that the sensor is turned on: press the round membrane button on the sensor several times
Finally, move the opening sensor and its magnetic contact closer and further away from each other to check their operation and the change in status.
If the problem persists:
- Make sure that there are no obstacles between LINK and the sensors that could prevent correct communication between the two devices
- Please make sure LINK is updated to the latest version.
You can check it this way:- Open the 1Control app
- Go into the app settings
- Open the "Manage your devices" section
- Select LINK
- Press "More options"
- Go to the "Update your device" section and follow the instructions shown
- Make sure the LINK antenna is screwed in securely and pointing upwards.