I need to change the mini-PCIe modems installed in routers RB912 to new USB modems Huawei E3372s. Surprised to find that they are hangs in a few minutes of work although routers RB951, RB711 work perfectly (even with 10m USB cable). I did a lot of experiments and concluded that c 99% probability the reason is that the RB912 has insufficient amperage for USB. Therefore, the question was really hard to design the device in such a way that, had it had sufficient power to the USB port? 
usb optimal length its around 4 meters.
try using a externally powered usb hub
check the integrity of usb cable
Thanks for the advice but with usb hub I can’t do usb power reset by router. The only problem is that usb amperage on this router is inadequate and no a word about this in the description. Moreover there were problems with some MiniPCIe modems so I think aperage is inadequate for PCI bus too.
To sum up Mikrotik still do not have the required device for users like me (and there are not so few).
- Beautiful & functional outdoor enclosure like at RB912
- Built-in WiFi transmitter 2.4G
- Ability to connect majority 3G & LTE modems through USB without any problems with power (even if required a power adapter with much amperage for this.)
I am very upset.
The problem got out only after I started testing at high speeds. Certainly try to write to support but hope for salvation is very small.
looks like a general problem of rb912
some users have posted issues with 802.11ac pci-express cards causing reboots and crashes