@Damian77 Hi, 

Thank you for your detailed info.  For your case, you could refer to the following instruction:
1. Check whether other devices, like a mobile phone connected directly to the router's Wi-Fi could get internet or not; 

And please check “ DHCP enabled” is Yes or No?  (Habitado Para DHCP in the photo you attached) 

2. Remove range extender or other APs and connect your computer directly to main router

3. Please check if there is any access control, MAC filtering  or special settings on the main router;
4. Please  try to ping the IPv4 default gateway and send us a photo and here is an FAQ:  https://www.tp-link.com/support/faq/425/
If ping succeeds, please reboot the computer and check the connection again.

May I know the model of main router and your network topology? Please help us check the driver version of adapter and computer motherboard info and send us the photos: https://www.tp-link.com/support/faq/2722/

Thank you!