I have a powerline extender AV600 TLWPA4220. I have paired the two parts successfully, connected the main one to the router via ethernet and can see and enter the wifi network of the extender. Although, when I join that network, I get the message that there is no internet connection and I am unable to go online.

I have tried resetting, matching the IPs from the extender's settings, even buying a new one and trying that out, but I always get the same problem. 

All lights in both parts are green, exactly as they should be, I see the TPLINK wifi network, I'm able to join but there's no internet. Same problem in both old and newly bought extender.

Any advice around what might be the issue here? Could it be a something from my router that I'm not familiar with? I've been trying to fix this for days and it's impossible and I really need the extender for my work.