I am (still) trying to troubleshoot phantom data usage on my wwan/qmimuxN interface, seems to be a failover / mwan3 problem. While doing so, I found that the darkstat package will not start properly when configured for a non-default interface such as qmimux0.
I did find a workaround:
vi /etc/init.d/darkstat
change line 41 to:
network_get_device ifname "$interface" || ifname="$interface"
see