Hello,
Since FW 7.0.0 the have been changes to the commands regarding data collection and the command for data usage now would be:
ubus call mdcollect get '{"iface_name":"<interface name>","conn_period":"<day, week,month","sim":<1, 2>}'
An example for this moths data usage for sim 1 would be:
ubus call mdcollect get '{"iface_name":"wwan0","conn_period":"month","sim":1}'
The values would need to added together and then converted to your favorite format.
Regards,
Paulius