I don't know what is your DHCP range on W8970, I suggest change IP address on WDR4300 for example 192.168.1.254 will be safe. And I'm interesting your address IP on computer.

DHCP disable on WDR4300. Please give us address IP, gatewaty and DNS form your computer (if you know command ipconfig /all in windows) and information what is LAN address on W8970,

No problem. Greetings from Poland.

Some routers have a problem with back conection, inside you can use local IP.

Everything is ok

You have to configure WAN as static IP 192.168.0.112 in TP-link. This configuration (port forwarding) is ok but you have to make reboot yours main router. If you like give me your public IP for PW, I...