@RE305-Extender 

Good day.

Thank you very much for your time and patience.

For your case, please help us check the following information:

1. When you mentioned while using a connection using an extender voice is dropping continuously, are you trying to say the internet speed would drop dramatically?
2. And how much speed did you get from the RE305, like 5ghz as well as Ethernet cable?
3. How far away from the router to the RE305?

If possible, please also try to modify the wireless settings on the router TL-WR840N, like channel 11 for 2.4ghz, channel width 40mhz;

https://www.tp-link.com/support/faq/2292/

Thank you~