Hello   @Loi 

Power Saving mode is ON on powerline unit by default, try to disable power saving mode  and see if lights stays on or not. 

How to enable/disable power saving mode on powerline adapters?

You could temporarily change the Wireless name of the wireless powerline unit to a different SSID from wifi router's and later connect network device to powerline unit then check internet. How to customize Powerline ac Extender’s wireless settings via web-based management interface?

To check what devices are connected to wireless powerline unit, you could log in web page of Wi-Fi powerline unit and connected clients will show up on status page.