I am having all sorts of problems with this board. I have tried 2 different USB keys.
1st key ZTE MF112, resulted in USB port invalid errors and random reboots
2nd key Huawei E169 USB modem 19 times out of 20 they board doesnt pickup the device after a reboot. I can disconnect the key a 100 times, plug it back in and it wont see it - Also now seeing random reboots
Only time I can get it to pick it up on random occasions is a power off for 10+ min or doing a fimrware upgrade or downgrade.
I have tried firmware 5.1 and 4.17
Is there any way of getting this to work and remain stable? I cant afford to keep on buying these keys 
did you tryed usb power reset feature?
when I run that command it says it isnt supported on this hardware
I have the same issue with this routerboard : rb433UAH.
I tried with several Huaway 3G modems and ZTE modem. All hardware supported, I specifically checked.
I tried with routeros 4.17, later with 5.2 and 4.13. No successfully results.
Is it posible that rb433UAH routerboad has any issue?
By the way,
[admin@MikroTik] /system resource usb> power-reset
ERROR: USB power reset not implemented on this hardware
I send the support.rif to be analysed.
supout.rif.zip (211 KB)
I have the same problem. RB411U w/ RouterOS 4.17 and latest firmware (2.29)
Pease tell us if RB411 can support USB POWER OFF mode?
ERROR: USB power reset not implemented on this hardware