Did you try to input APN and dail number manually? http://www.tp-link.com/en/faq-400.html

Go to Forwarding---UPnP and disable it. It may help. If not, contact TP-LINK support team on this issue as it is a replacement already... http://ticket.tp-link.com/index.php?/Tickets/Submit

Is it WA854RE or WA850RE? Firmware is up to date, right? Any other wireless client works when connecting to the range extender? The TV can connect to main router's SSID, right? What's the IP address,...

If upgrade won't help, you can save config files and send them to TP-LINK support team for further analysis. http://ticket.tp-link.com/index.php?/Tickets/Submit

But Vista W8.1 and W10 can connect to main router's Wi-Fi name and work properly? Can you update driver for wireless card on those devices?

Did you try what I said? If you select the type of Smart IP (DHCP) and there is a DHCP server forward, the DHCP server in this device will not start up. Otherwise, DHCP server will start up.