After a few weeks of operation, I had one of the Huawei 909s-120 go down and not reliably come up again. First updated the Huawei LTE-Card firmware - no satisfactory result. The upgraded RouterOS to 6.38rc28. Now the LTE-Interface comes up reliably, but stops working after a few megabytes traffic (~50MB), just as reported in this thread here
RB912UAG with LTE Card Huawei ME909s-120 stop working after 50MB Traffic