On web interface of WDR4300, go to Wireless 2.4GHz---Wireless Statistics page. Can you find out MAC address related to TL-WA830RE there? Try to add that MAC address in MAC Filtering rules.

Not sure but can you have a windows update or something like that?

C9 supports Three-Address Format WDS. If MR3420 works as the main router and C9 works in WDS mode, it should be OK.

There is a beta firmware to solve chromecast issue indeed but you have to get it here. http://ticket.tp-link.com/index.php?/Tickets/Submit

orignal router(modem?)---second router(wireless router?)---PA511====== PA511---PC(error code 651?) Is my above understanding right? On your PC, there is broadband connection? Because as I know error...

What's the operating system of your laptop? Are there any other wireless clients for you to test? Channel width is 40MHz? Network is secured with encyption AES? Can you post some screenshots of your...