Think in terms of your local network rx/tx traffic and not rx/tx traffic between your gateway ip and your ISP.
Bandwidthd uses a libpcap format filter string to control what bandwidthd see's, so you really shouldn’t capture your gateway and broadcast traffic statistics.
Try changing your /etc/config/bandwidthd option filter setting to option filter='ip and not host 192.168.1.1 and not host 192.168.1.255’
This will just capture your local subnet clients rx/tx traffic which for the most part is all inbound/outbound traffic through your gateway to/from your ISP.
EDIT: I should have mentioned this community member initiative to track/restrict clients via quota using Bandwidthd.