Reset the deco. Configure it when it's flashing blue. 

 

If that failed too, use another Deco as the main one, then add the other Deco. 

 

When adding another Deco, reboot the main one; or connect the new Deco to the main one by Ethernet cable. 

 

FYI

https://www.tp-link.com/en/faq-1447.html

 

 

@Anton L.Z. "adding new Deco" in my post includes "reset" and "wait for flashing blue". "Fails" means "configuration fails". Have tried also - to connect this deflect Deco directly as stand-alone ("setting up this device as own network") - to connect this Deco to two different Deco networks (not only Master Deco changed but the whole network starting from the modem and ISP provider) - to use cables for connection Took device back to store; they suspect device is deflect and sent it for replacement.

All right. 

 

May contact tplink support to confirm it.