Welcome to the Community!
The usual WiFi range of the standard modem/router is 30 meters line of sight (no interference / walls).
Since you are not able to connect to the network if you are 15 m away from the modem, then we need to check the thickness of the wall in between and other factors that may interfere with the WiFi signal.
We've created some articles that may help improve your home WiFi network.
You may also consider getting a mesh network or an extender to minimize any black spot in your premises.