@Praneeth  Hello, 

Do you need an extender wirelessly or wired to router to extend network? 

If you are using A6 V2 and above hardware version, you could consider a  dual-band oneMesh compatible range extender with AC1200 or above Link speed.

If you use RE in AP mode wich is wired to router, RE with a Gigabit Ethernet port would be better like RE505X V1 , RE450 V4 etc.

https://www.tp-link.com/onemesh/product-list/#range-extenders

@Solla-topee I am looking for Wireless. Current Router which i am using is TP Link Archer C6 AC 1200. Please assist which wireless extender i need to purchase for better signal within building. You can also suggest for new router as well to cover entire building 4BHK apartment.

@Praneeth Hello, 

Router may have difficulty wireless bridge to each other, I would recommend extenders more.If your house is under same power circuit or under same power meter, considering you have multiple rooms (walls)

you could consider some powerline products:

Common Topology look like below, you could add more powerline wired extender or WiFi extender and pair it with the main Powerline units. 
PLC: 
Router --<Ethernet> main Powerline unit ______<house powerline> (Wi-Fi)extender Powerline unit <wireless or wired> Laptops/ phones etc. 

Main Powerline unit ______<house powerline> # 2nd  extender Powerline unit
 

If ISP bandwidth is more than 100Mbps, you could consider AV600 like model WPA4220 KIT; more than 100mbps bandwidth, you could consider AV1300 like WPA8631PKIT

A kit contain a wired unit connected to router, and a wireless units work as an extender, and you may need more than one extenders. (but they normally sold by KIT, only one has WIFi function)