On your main router, channel is fixed as 1? How did you check the current channel of RE200? On Status page of its web interface, what's the channel displayed under 2.4GHz? Provide some screenshots....

So it needs portal authentication to access internet. As I know, TP-LINK has no devices which can share such kinds of network...

An ethernet cable is connecting the router with a patch panel, wiring rj-45 connections in different rooms. It seems that you need an access point. WR841N can be configured as an access point. Check...

You are trying to set up the router to work with Virgin media cable modem? http://www.tp-link.com/en/faq-608.html

We can enable WDS on C9 to connect to main router wirelessly. If you want to use the same SSID, you can change SSID of C9. http://www.tp-link.com/en/faq-825.html

D7 allows command line access via telnet as well. You have to figure out all command lines supported by D7 via help command by yourself.