@MichV 

The Deco should not be flashing, only a constant green LED light.

Swap the main Deco unit for another, and designate that as the new main Deco. Look underneath to reference the right MAC address.

If the main Deco unit started blinking then likely it was no longer providing an internet connection or local connectivity. 

You could try resetting the blinking unit by leaving it powered on for a couple of minutes then pressing the reset for 2 seconds. 

If I want to setup a Powerline extender, do I need to use different SSID from my main router or can I still use same SSID and passowrd as my main router?so that I don't need to manually reconnect again when I move from main router area to powerline extender area next room? Thanks

@TLWIN 

You can have the powerline use the same SSID.

I would recommend having them be different so you know exactly which network you are connecting to.