@ME1975 

Hi, Thanks for your time and patience.

Here is some information for your reference:

1. Do I need to do some config on the Asus router to have two Deco Behind working in router mode?

Nope

2. Why does the second Deco show zero traffic even if the clients clearly shows traffic? 

You could wat for a while and sometimes the App needs time to update the data. And based on the feedback I have seen in the community, most of the time, the real-time speed rate here is not accurate and is just for reference. If you do want to know the bandwidth of the satellite unit, it is suggested to connect a computer via Ethernet cable to the satellite unit or connect to the 5ghz wifi of the satellite unit.

Thank you very much.

Best regards.