@Jennseay 

The Deco needs to be initially set up with a TP-Link ID in order to actually get to the internet. If someone connected a Deco to your modem, and it went green right away then it likely was set up with another account already. 

What I would do is reset every Deco node by holding the reset button for 2 seconds. Once the nodes start blinking blue you can then set them up one by one. I would probably just have one powered on (wired to the modem), set it up through the app, then power on the second node, and so on.

If you created a TP-Link ID already, then you would stay logged into the app, and then hit the plus symbol on the top right to start adding the Deco units.