Download the fing utility from fing limited to your phone. We connect to Wi-Fi, scan, get a list of devices. We compare mac addresses and models with our devices, if we find a new one, we change the password for the Wi-Fi.
Ensure that all your devices are bound to the router by the mac address and the network can be used without a password at all, other devices that have not been identified by the mac address on your router (according to the created rule) will not be able to connect to the network and use it.
BackBox, Caine linux, WifiWay, Wifislax, Parrot Security os, then everything is in your hands. Designed for penetration testing and security assessments. However, this can also be used for hacking.
First, you need to find out if someone is really connected to you. If you really find a thief, then the easiest option is to set up a MAC filter on the router and change the wi-fi password.
Here's a article with detailed instructions to help you - https : //lanportal.ru/wifi/kak-uznat-kto-podklyuchen-k-moemu-wifi.html
The main way to protect Wai-Fai from neighbors is not to give them a password from the network. Otherwise, everything is simple - make the password more complicated and set the most reliable encryption standard, as advised here https://192-168-0-1.ru/zashhita-routera/.
Hide the broadcast of the SSID as here, I advise some to no use, since this will not help and such a network will not be detected only by a completely illiterate person. Defend yourself wisely and you will be happy!
Hide SSID then your point will disappear in any phone or laptop scan. Then rename the SSID then your WiFi will definitely not be seen or found by the old name. Enter yourself through the dot name.
Very necessary !!!