Thanks @RobinWinbourne I just removed the ST multi sensor from the mounting bracket and bought it and placed it near the garden door which is about 4 or 5 meters away without doors from the ST hub and ST seems to have picked it up again. So wondering maybe I should mount this on on the garden patio door. It was originally installed on the front door which is about 8 meters away with a interior fire door in blocking it which is always kept closed.
When I was originally trying to discover this sensor from the front door, it just wasn’t picking it up. I had to bring the sensor close to the ST hub for it to discover it.