Did you refer to DDNS settings? What DNS service provider do you use?

Did you contact your ISP to check the line quality? http://www.tp-link.com/en/FAQ-547.html

You may upgrade firmware of the extender to the latest version first and reconfigure it. http://www.tp-link.com/en/download/TL-WA850RE_V1.html#Firmware If it won't help, when internet stops working...

To repeat Wi-Fi signal from the second range extender, the connection may not stable enough. Powerline extender should be proper in your case... You can buy one TL-WPA4220KIT(composed of TL-PA4010...

Try to change primary and secondary DNS to 8.8.8.8 and 8.8.4.4 on DNS---DNS Settings page.

Andrew, WLAN Autoconfig is enabled or not? http://www.tp-link.com/en/faq-306.html