@Drshn By repeater mode did you mean you're using Archer C6 in AP mode. If that's the case take a look at the difference between wireless router mode and AP mode here. In this case some of the device...

@cootle If your TL-WA855RE is HW v5 try the beta FW from this thread.

@AllanM You could use the http://tplinkplc.net to open the PLC web management page and check the HW and FW versions. Adding a range extender to your wireless network shouldn't be a problem.

@cootle Reset the RE to its factory defaults and start adding it to your wireless network using this guide.

@Mraajaj When you're connected to the range extender wireless SSID try reaching its web managment page via http://tplinkrepeater.net.

@Noswal You should be able to connect a second RE to your main router without any problem - try this guide.