@Roy22 

What is the current network diagram? 

As you can see, the Deco unit should be solid green if it works well. Solid blue means that the Deco is setting up.

https://www.tp-link.com/support/faq/1464/

Back to your case, please connect the slave Deco to the main Deco directly first to make it works first; in other word, please confirm that you can access internet from it.

After that, you can put it out of the range from the main Deco and check its led status and the network connection.

If still the same, please connect your computer to the slave Deco and click here to verify its IP address and gateway. 

Best regards. 

@Roy22 

Hi Roy,

Thanks for your reply, glad to hear that you figured it out finally. Congrats. 

I did notice that you start another thread about the firmware upgrading, but not able to bundle them together.

Anyway, the Deco network is working properly now, you can enjoy the internet with them.

Good day.