I suggest you to send an email to TP-Link Tech Support about this issue so they can have a look into it.

Also check the LEDs and line status on the status page, you may check whether the line is good according to this link: http://uk.tp-link.com/faq-546.html

It sounds like that your Android phone is not connected to the same wifi with the smart plug, or there are some settings on the router which blocks the communication between them. You may check which...

If you use another wireless device, like laptop, tablet, to connect to the TD-W8961ND, will it cause the same issue? Who is your ISP? Have you tried to change wifi setting like SSID, channel, channel...

Here is the instructions for WDS Bridging: http://uk.tp-link.com/faq-447.html , but this function may not work well with the main router sometimes. You may have a try first.

What's the old firmware and new firmware of yours? And what's your issue with the new firmware? You may give the details to tplink support(support@tp-link.com) to get help.