yes, that was it. I somehow figured it out when I saw a new MAC Id when I connected then without any security under the wireless statistics and added it to the allowed list. thank you for replying.

Model : Hardware Version : Firmware Version : ISP : I have 2 Archer C20 v4 with 170822 firmware. One is connected to the WAN upstairs and other is connected to the main router by Wi-Fi as an extender...