First of all, I would highly recommend updating your device to the latest firmware, as it may contain changes that include optimizations and traffic corrections.
There is a way to check the traffic that's happening not only between your connected devices but on the router itself.
To do that you will have to navigate to:
Status > Graphs > Connections
Here you will be able to see all incoming and outcoming traffic and calculate it against mobile traffic shown in the overview page.
If you have RMS - disable the connection to it, your router may also send constant data to RMS and it would mean more data traffic than it should be.
Once you go through all active services that could be sending or receiving data and make sure they're disabled - you should be getting significantly less data usage.
If you're still worried that there's too much data traffic you can always use QoS functionality, which will let you limit not only your default interface data traffic, but also application data usage determined by ports.
Also 20 - 40 kb data usage every 15 minutes is not very significant (~100MB monthly)