@Mandana 

You could put RE220 in the halfway between WiFi and dead zone, and see if signal become stronger with RE plugged in, the strong WiFi normally comes from RE. 

Or you could set a different SSID from router  for range extender, so we could distinguish the WiFis coming from router or RE.

( change Wireless settings via Tether APP) https://www.tp-link.com/support/faq/2541/