Using tiles integration

Hi, hopefully this is a simple one.

Before I buy some tiles tags, I wanted to know if the tracker monitors location by connection of the tag to the device running the app, or purely gps?

I have a room at the bottom of my garden, too close to use gps reliably and on same WiFi network but want to know specifically when someone enters this room (by attaching a tile to the key to the room).
Not a programmer so looking for a simple solution if this won’t work.