I had the same problem, and (I hope) solved it by disabling WLAN.
Mikrotik, is it possible that this bug still exists?
I use ME909s-120 with mPCIe to USB converter on 2011UAS-2HnD with Ros: 6.44.3
I've tested ME909s-120 with FW version: 11.617.15.00.00 and 11.617.09.00.00
but i obtain "lte1 link down" after a few Mbyte downloaded!!!
If i disable wlan all work ok! But...i doubt that is an insufficient power supply because for the final test i've used an special "splitted usb" cable that power the usb LTE devices by an external power supply... But SAME PROBLEM!
Well we need to investigate more accurately, because in the first day that i've inserterd the lte usb all it works!... one day suddenly give me this strange problem!.. hope that it help