This question already has an answer here:
I'd like to monitor my network traffic outside the local network on my main computer (a gentoo linux desktop workstation), which goes through a 3g wifi router (a Huawei e5220); this is a wifi only device, I have to connect to it directly through wireless connection, and I can't install any monitoring system on it.
I have a 10gb per month limit, but to check it I've to go on my mobile operator website and browse through 5 pages, and the web interface of the Huawei doesn't show reliable statistics. Since sometimes I have file transfers within the local network (a notebook, my mobile phone), the statistics shown in conky through totalup/totaldown or even the ifconfig's RX/TX output are not correct, since I'd just like to have the sum of R&T traffic outside of 192.168.xxx.xxx.
I know that the statistics will not be precise (using other devices that will connect to internet through the 3g router, I'll never know the exact amount of used traffic if not by going to my operator's website), but an approximate value would be enough.
I already checked out other questions about iptraf, ifstat, wireshark and other tools (I don't want a graphic monitoring tool, I just need a simple output text I can use with python/bash scripting), but none of them seems to answer to my situation.
EDIT: This is not a duplicate of Monitoring Bandwidth Across Multiple Systems in a Home Network: as explained I can't change the firmware of the router and it's a wireless only device, so I can't use another computer or wifi router to monitor the bandwidth.