When I install the thunderbolt Ethernet adapter with it connected to a switch the switch shows the gigabit connection with the LEDs but the MacBook doesn't show the connection as being active. Looking at the Mac under System I see Thunderbolt 1 and 2 as being inactive. OS is 10.12.6 Beta and HW is MacBook Pro Retina 15 inch Mid 2015 Any ideas?
Go into System Preferences --> Network --> on press "the tiny wheel" (see attached image) --> Set Service Order --> Re-arrange "Ethernet" to be above "Wi-Fi"
I had this issue too, where everything was showing that it's connected and working. But I was still getting slow Wi-Fi speeds. Re-arranging the Service Order, helped me.
Hope it helps you too.