Why is ONLY my Wi-Fi not displayed?

I'm visiting my mother with my brother, I do not see their Wi-Fi on my notebook or smartphone. All other Wi-Fi signals from the neighbors easily 15 pieces are also shown to me. My brother was able to connect to the notebook and smartphone without any problems. My mother's guests also worked without any problems. In my home, both devices are also connected via Wi-Fi. Someone have an idea what's going on.

Is it possibly set in the router that the SSID is invisible?

Switch the router off completely and on again.

The same with your devices.

Try to set up the WLAN manually on your device