Data Usage Meter Problem

I recently installed Openwrt on my router
but Because It's wifi is only 2.4 Ghz the wifi speed is low on it. I get the Internet from an LTE Modem which has wifi itself and I connected it as WAN Port to Openwrt router. Now the problem is that when the devices connect to the LTE Modem WIFI the Yamon (Data usage Meter tool On Openwrt router) does not count their Data usage. Is there any solution for this That I can do sth that every client on the whole Network no matter which Device Wifi they are connected to it counts their Data Usage?
And If No Is there any other Data Usage Meter Tool than can do that?

How about to disable the Wifi on the LTE modem?

The Wifi is very weak on OpenWrt router because of only 2.4GHz Freq available so it is the only option

If you want to keep taps on your data usage, all data must pass through the router (so wifi disabled on the LTE modem). Maybe consider getting an OpenWrt router with better wifi instead.


How about that
I create a PPTP VPN server on openwrt modem and connect the LTE modem to that server as a client.would it do the job?


If I use another wifi router and connect it to openwrt with Lan Cable
Would it count the data downloaded by that clients connected to that 3rd router?

That could work.

If you can highly configure your modem: You could play around with the routing table, like route all traffic from the modem's wifi to your router and from there over lan back to the modem.