see here, http://www.tp-link.com/us/faq-468.html if no luck, you can look for powerline adapters. One goes to router and the other Wi-Fi one upstairs to send signal. Make sure they are in the same...

try reset first and if that doesn't fix, ask the shop or TP-link to provide you a replacement. it seems that this router is not working after firmware upgrade. By the way, do you have dell laptop...

do you want to use it as range extender or wireless access point? can't get started, do you mean no light at all? have you login its setup page?

how do you connect them? A topology may help us to understand the issue here.

which configuration you want to change? wireless username and password? it has a web management page you can configure it. http://www.tp-link.com/us/faq-87.html

not following you,sorry. do you mean without extenders C2600 works fine?