Hi,
I recently bought a new Mikrotik model RB951Ui-2HnD and inserted an LTE Dongle "Huawei E3531" into it and everything works fine until some point the Mikrotik start to lose the communication with the dongle and lost the DHCP IP got from the dongle until when I reboot the Mikrotik it starts to communicate with the dongle again.

I tested the last stable and long term firmware for the RB951Ui-2HnD and with the 6.48 (Stable) it crashes in minutes but with 6.46.8 (Long-term) it takes some times to lose the connection with dongle and it depends how much you downloading the data from the dongle.

I have an older model RB751U-2HnD with the stable version of firmware and it's working perfectly with the dongle, even the download speed of the dongle sensibly better than the new model.
I inserted the HB USB Hub to the new model to help the increase of the power usage of the dongle may be the Mikrotik can't handle the current peak of the dongle but makes no difference.
Thanks for any advice.