I recently flashed openwrt on my TP-Link Archer C6. So the basic setup is as follows -
Public Wi-Fi ------> Open-WRT 5Ghz (Client) --------> Open-WRT 5Ghz (Virtual interface) (WLAN) ------> Multiple devices
So basically the client and WLAN are same physical adapter and are split into STA-AP mode. The public wifi uses captive portal to recognise the mac address of the device when you login for the first time using the credentials. It saves the mac-address and you won't have to repeatedly login. So everything is logged in. I can use internet properly on my PC with WLAN. However when I connect to the TP-Link router using my Android Phone it always says sign in required and it would try to redirect me to the captive portal page. However the captive portal (login.myion.in) doesn't open. Instead there is an option in the browser to "Use this network as is". I click on that option and the internet works fine for that session. However when I reconnect again then I have to repeat the same process. I just want it to bypass this captive portal detection as the internet is already working. I still don't understand why its asking to sign in when the internet just works fine.