Bandwidthd behind another router

Hello,

I want to put a router (a GL-AXT1800, from now on called "the slate") behind another router ("the router"). The slate has OpenWRT and I have full control over it. The router will be someone else's router that I can't modify. Client devices are connected to the router, like in the diagram below.

modem <--> slate <--> router <--> client device

The reason for this setup is that I'm doing an intervention study in volunteering peoples' homes. I want to change their network configuration temporarily and easily, ideally just rewiring some ethernet cables.

What I want to do is use bandwidthd (on the slate) to log bandwidth usage per device. I've gotten this to work when the devices are connected directly to the slate, but when it is put behind the router all data is seen by bandwidthd to come from the router (i.e. bandwidthd can't tell the devices apart). My understanding is that this is because the devices are on a different subnet, so from the point of view of the slate it all comes from a single ip address (the router's). Is this correct?

How can I make bandwidthd tell these devices apart?

yes

swap router and slate.
or remove router, but I guess it's not an option ?

swap router and slate.
or remove router, but I guess it's not an option ?

Yeah not really an option. Or, it's a messy option since that would mean that I would have to reconnect all devices in a household.

Is it possible to configure the slate to be on the same subnet somehow? afik the slate doesn't need to do that many network things except acting as a switch.

reconnect how ?
just change the SSID of the slate's wifi to match the one of the routers ... ?

it is, but I'm not sure bandwidthd will work, if you do.