Use this script from the post of accord (post 16).
http://forum.mikrotik.com/t/interface-data-quota/79514/17
This script (slightly adapted to your interface) works great!
It should achieve what you want.
It is not very well documented/commented, but just copy it, and change the interface name to your LTE interface.
Then see the L7 firewall values if the data rate is measured.
I run this script every 1h (as I switch LTE on and off when needed), but if it runs 24/7 you could do it only ones or twice during the day
(it is writing to FLASH memory as values remain after reboot).
Don’t use the more recent scripts in that topic (I have not tested it).