Bandwidth monitoring on bridged dumb wifi AP

Currently I have FTTH modem + wifi router to access the internet. However, my internet is metered and I want to monitor bandwidth usage per device (per LAN IP).

So I have a spare Raspberry Pi 3, I configured it to bridged dumb AP mode (eth0 bridged to the wlan interface), and clients can connect to RPi3 and have the DHCP from the original wifi router working and able to access internet. Basically the setup is this:

(clients) ---->[wlan0] RPi3 [eth0] ---> Wifi router(with PPPoE) --> fiber modem

I installed the nlbwmon with the luci interface but it doesn't seem to be able to capture the traffic going through the eth0. All it capture is the traffic to the RPi3 itself. I fiddled with the nlbwmon and doesn't seem to work anyway. IPv6 traffic in the nlbwmon also appears to be empty.

The question is, does the nlbwmon work for bridged interface? Do I need to set it up as a router instead?