If it is telephone line, is it VDSL or ADSL? Do you want to replace the openreach modem with TP-LINK or make them work together? With VDSL connection, as TD-W8980 is not capable of it, it cannot...

How about status of all lights on your powerline adapters? What's the model number of all of them? Did you pair them? http://www.tp-link.com/en/faq-258.html Wired connection to Wi-Fi powerline...

All wireless clients are able to connect to main router's Wi-Fi when range extender is off? The range extender is using the same Wi-Fi name as the main router? Try to remove network profile or forget...

You are right. WA860RE is not a dual band range extender and it cannot broadcast 5GHz Wi-Fi signal.

I think it is enough to configure wireless mac filtering as wired clients have to connect by cable and no password is required. http://www.tp-link.com/en/faq-324.html If you want to block wired...

It is going to be supported but not sure when. You may keep an eye on their website. http://www.tp-link.com/common/promos/Compatible.html