Thunderbolt Ethernet not working on MacBook Air

Hi,

I am unable to connect my MacBook Air (Yosemite) to internet via my Thunderbolt Ethernet cable.

I plug in into my internet provider box (working), and it is not even "seeing it".

It's written "inactive" in the status.

I tried with my Windows laptop and it works perfectly (ethernet cable and internet connection), eveytime I connect it.

Here's what I did after reading the forum:

-turn off the Macbook, unplug the cable, start the MacBook, plug it back, many times

-refresh the IP address in DHCP, many times.

-updating the the adaptor (it was already updated).

-PRAM reset

-SMC reset

-changing the ethernet port on the box (lights are blinking on the box, but nothing detected on the Macbook

I am missing something or is my thunderbolt to ethernet adaptor (Apple) defective?

Thanks in advance for your help,

Cédric