Hi  @Stepanmanak 

When you don't see the repeater's Wi-Fi under the Wi-Fi list, you can check the signal LED light to confirm the connection status:
1). If the 2.4G/5G/Signal lights all on, the extender is connected to the main router. The range extender might use the same network name as the main router, so you cannot see it separately. You can relocate the extender between the dead zone and the router and see if you get a better signal now. Or you can change the extender's Wi-Fi name to something different following this guidance:
How to change wireless settings of TP-Link range extender on Tether APP

2). If the signal LED does not light up. Please try factory reset the range extender and check if you can see the default TP-Link xxx Wi-Fi under the Wi-Fi list. Then, try to reset the extender and follow the step-by-step guide to set up the extender: 
How to configure Range Extender via Tether APP