You tried to connect to main router's Wi-Fi or other Wi-Fi networks to test if the Whatsapp problem continues?

How about status of 2.4GHz WLAN light? SSID broadcast is enabled? Did you try to detect it by any other wireless clients? Change the channel to 1, 6 or 11 and try again.

Upgrade the latest firmware and follow the guide below to protect your router from potential attacks. http://www.tp-link.com/en/faq-573.html As to feature request, you'd better submit it here....

Yes, it does. You can check its specifications here. http://www.tp-link.com/en/products/details/cat-15_TD-W8960N.html#specifications

Wi-Fi password of your main router contains Ñ? If so, I'm afraid that you have to change it to an ordinary one, so that you can configure the range extender.

You tried on different wireless clients but motorola tc55 is the only one having the problem, right? Maybe it is the problem of wireless card on motorola tc55.