Hi,

First, connect your computer to the PoE adapter directly to test whether you can access internet; meanwhile. bypass the PoE adapter, connect your computer to the airgrid directly to test the network connection. 

Note: Please check their IP address and default gateway. https://www.tp-link.com/support/faq/838/

If there is no internet access, you need contact their tech support to confirm the line connection. 

While if the computer works fine, please connect the computer to the C20 by wired cable and login to the web interface to clone the mac address. 

Note: Login to the web UI first, and then go to advanced-network-internet-mac clone page to do so. 

Besides, you can try to configure C20 as an access point. Below is the instruction: 

https://www.tp-link.com/support/download/archer-c20/v4/#video

May it help and have a nice day.