It seems the extender has been reset to factory default by accident. On this case, need to reconfigure it. Refer to this link to set it up again: http://www.tp-link.com/en/faq-357.html

Not familiar with DD-WRT, maybe you can contact them and see what's happening.

Do you connect the second extender to the first extender's wifi? If they connect like that, the connection will be unstable and gets slow speed. Normally just connect both of them to the router, and...

Sometimes the hotline is really busy. For your issue, it is better to send email to support.ca@tp-link.com. It is a strange problem, the technician may need to test on it and follow up in email. BTW,...

Can send email to support@tp-link.com, they may need test on it

Did you change Ethernet cable and ports on the router? Or please check if there is any IP address on your computer.