In your modem/router there’s mostly of the time a list of connected users. If in that list are devices not known by you it is possible to block those MAC address shown in that table.

For us, without the knowlegde of your modem/router brand its not easy to tell you where to look.

There are small programms/apps for the PC that scan the network you're on. IP scanner or Softperfect Network Scanner are some examples.

@Bhaskar

I have a Zyxel and when you log in on your modem interface click on verbondenheid and under WiFi youll see all devices which are connected, unknown devices can be blocked by MAC address as Yalerta mentioned.

Make sure you don't have an easy password for your WiFi and you could setup a guest network for guests.

Thank you both @yalerta  @Waqqas . :)

I have a Zyxel also

I have created a user id to login , But can I see the connectedness  tab if I login with my username and pwd to T-mobile thuis?

Or I need to login only modem interface?

Because when I tried login modem via http://192.168.8.1 , I am getting site cant be reached error.

@Bhaskar

Its 192.168.1.1, username is admin and password is on the back of your modem. After logging in click on verbondenheid.

Hello @Bhaskar 

The first thing i noticed is, that you are using an incorrect IP of the TM-router.

The adress should be http://192.168.1.1, that will bring you to the interface of the router to login with the details that are mentioned on the sticker at the back of your router.

Login on your router with this data.

When you are logged in, you have a box with on the right side bottom a triangle shape. Click this portion to go to the details of all connected devices.

On the new page, you will see under the Wifi tab all connected mobile devices to your router.

If you see there devices that belong to strangers you might think, my first advice is to change the password of your Wifi. The standard default one is mentioned on the back of the router, but it is wise to change this to your own strong password of at least 8 - 12 characters (also incl @!$ is possible)

To change your Wifi password, go back to the main page and select the right lower corner inside the Wifi-instellingen.

Thanks again @Waqqas 

It worked! Have a nice day!