Not supported for now. LuCI can see the traffic statistics of the interface, but it will deviate from the operator’s data.
Later releases of the 4.x firmware will show mobile data usage for 4G LTE. The date is gotten from Modem, but we can’t ensure that the data is exactly the same as the carrier’s. The best way is still to check with your operator, if you want to avoid traffic overload.
The limits or reminders of 4G LTE mobile data usage are still being discussed.