Because MikroTik as vendor not share that info 
I read a many of AT Manuals and that way I sometimes know someting and try help in all LTE posts here.
First the latest firmware for lte module, my status list:
- R11e-LTE12 (EG12-EA) EG12EAPAR01A09M4G
- R11e-LTE6 v027
- R11e-LTE MikroTik_CP_2.160.000_v018
- R11e-LTE-4G v9
- R11e-LTE-US v16.02.183961
- SXT LTE 3-7 |-> chip:2.0 fw:0.1.2.4
Probably you write about R11e-LTE MikroTik_CP_2.160.000_v018 and if you have some problems with upgrade firmware then write to support directly.
That codes I remember in two examples:
a) firmware upgrade solve problem
b) user have problem with ISP and after change IMEI they could connect.
For me that case not have proper logs who show the situation.
Enable the lte topic in logs and this
Watch LTE parameters: http://forum.mikrotik.com/t/r11e-lte6-2ca-not-staying-connected/136004/22
give way to track what happens here.