Thank you for reaching out!
Instructions on how to use JSON RPC with the curl command can be found here.
It depends on what information is needed, but to get mobile data usage, commands like gsmctl -e mob1s1a1 (number of bytes sent) and gsmctl -r mob1s1a1 (number of bytes received) can be used.
Please keep in mind, that this will only work up to version RUTX_R_00.07.02.7. Later versions use different API calls.
Let me know if you experience any difficulties!