http://www.tp-link.com/en/faq-1563.html

That's rare. It's better you contact tplink support directly, email them

Hmm..? Your son is not connected to the Deco? He is connected to a powerline adaptor? What's the network map? So you tested with your iPad, the APP still showed the data flow when the iPad was...

Red means the powerline signal is not good, too many interference in the electric wire between the two adapters. Did you try plug the two adapters in the same room to pair them again?

What's the link speed shows on the tpplc utility? Did you test the speed when the adapters in the same room?

Try setup a different Wi-Fi name on the AV600