Hi Automan, provide the detailed topology(how did you connect all devices). Back up the current settings, save the config file and send it with login username and password to TP-LINK support team for...

They are compatible with each other. Refer to the following guide to pair them. http://www.tp-link.com/en/faq-258.html You can connect the ADSL router to any LAN ports. They are the same LAN ports...

You didn't mention WAN IP obtained on router page. Is it public or private? Provide your topology pls. http://www.tp-link.com/en/faq-785.html

What's the operating system of your PC? Can you connect the PC directly to the router and check if it works? Maybe it is a driver issue of the PC? It seems that powerline devices are working OK...

In client mode, to access internet by WA801ND, you have to use cable connection main router(Wi-Fi)>>>>><<<<<(Wi-Fi)WA801ND(LAN)-----------(LAN)PC Did you try if it works wiring to WA801ND? If you...

Cannot do that. If the main network is open, the extender network will be open as well. If you want to secure the network, you have to secure the main network and reconfigure the range extender.