Monitoring bandwidth with luci-app-nlbwmon and two routers on the same subnet

This, at least if I understand you correctly, cannot work. Routing works by forwarding packets from one subnet to another, therefore the subnets (IP address ranges) must differ between WAN and LAN.

If I understand you correctly, all of your clients are in the same subnet as the ISP router and your tl-wdr4300, which means routing doesn't take place and most traffic bypasses your tl-wdr4300.