Region : Argentina

Model : TL-WA701ND

Hardware Version : V2

Firmware Version : 3.17.0 Build 140324 Rel.36563n

ISP :

Dear users, please help

I have a big problem, I'm using TL-WR740N Router and TL-WA701ND Acces Point at home. My root router TL-WR740N is on the 1st floor and the Access Point is on the second floor. I bought the AP to extend the wifi signal on the 2nd floor because it was weak. The AP is set it to work as Range extender ( Repeater ) and is wirelessly connected with the Root router. I use the setup wizard from TP LINK to connect this devices, but I'm facing with a big problem. When I'm on the 2nd floor and connect from a mobile phone to the network ( i.e. Access Point ) after 1-2 minutes my connection drops, and after that again it's connecting and so on, I don't have a stable connection. It's connect - disconnect all the time.
When I go downstairs my phone is connected to the Root router and the connection is great, no more connect/disconnect. I'm using the same SSID, same Wireless security and AUTO channel choose.

Please, can you tell me what is causing this problem, is that a problem that the Root router is on the 1st floor and the AP is on the 2nd floor and the signal between them is not stable or something else?

Thanks in advise :)

P.S. I'm writing from Macedonia, not from Greece.