Вставляем ZTE MF626:

загорается оранжевый индикатор

usb 5-8: new high speed USB device using ehci_hcd and address 3
usb 5-8: configuration #1 chosen from 1 choice
Initializing USB Mass Storage driver...
scsi2 : SCSI emulation for USB Mass Storage devices
usbcore: registered new interface driver usb-storage
USB Mass Storage support registered.
usb-storage: device found at 3
usb-storage: waiting for device to settle before scanning

через небольшой промежуток -- синий индикатор:

usb-storage: device scan complete
scsi 2:0:0:0: Direct-Access     ZTE      MMC Storage      322  PQ: 0 ANSI: 2
sd 2:0:0:0: [sdb] Attached SCSI removable disk

Делаю modprobe usbserial vendor=0x19d2 product=0x0031

usbcore: registered new interface driver usbserial
drivers/usb/serial/usb-serial.c: USB Serial support registered for generic
usbserial_generic 5-8:1.0: Generic device with no bulk out, not allowed.
usbserial_generic: probe of 5-8:1.0 failed with error -5
usbserial_generic 5-8:1.3: generic converter detected
usb 5-8: generic converter now attached to ttyUSB0
usbcore: registered new interface driver usbserial_generic
drivers/usb/serial/usb-serial.c: USB Serial Driver core

Запускаю sudo mf626-helper -v 005 008
I: Trying to open 005:008
I: Found ZTE626 modem

проходит ~9 минут...

I: Modem 005:008 has been disconnected. Terminating.

в dmesg:

usb 5-8: reset high speed USB device using ehci_hcd and address 3
usb 5-8: device descriptor read/64, error -110
usb 5-8: device descriptor read/64, error -110
usb 5-8: reset high speed USB device using ehci_hcd and address 3
usb 5-8: device descriptor read/64, error -110
usb 5-8: device descriptor read/64, error -110
usb 5-8: reset high speed USB device using ehci_hcd and address 3
usb 5-8: device not accepting address 3, error -110
usb 5-8: reset high speed USB device using ehci_hcd and address 3
usb 5-8: device not accepting address 3, error -110
usb 5-8: USB disconnect, address 3
scsi 2:0:0:0: Device offlined - not ready after error recovery
generic ttyUSB0: generic converter now disconnected from ttyUSB0
usbserial_generic 5-8:1.3: device disconnected
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: [sdb] READ CAPACITY failed
scsi 2:0:0:0: [sdb] Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK,SUGGEST_OK
scsi 2:0:0:0: [sdb] Sense not available.
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: [sdb] Write Protect is off
scsi 2:0:0:0: [sdb] Mode Sense: 00 00 00 00
scsi 2:0:0:0: [sdb] Assuming drive cache: write through
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: [sdb] READ CAPACITY failed
scsi 2:0:0:0: [sdb] Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK,SUGGEST_OK
scsi 2:0:0:0: [sdb] Sense not available.
scsi 2:0:0:0: rejecting I/O to dead device
scsi 2:0:0:0: [sdb] Write Protect is off
scsi 2:0:0:0: [sdb] Mode Sense: 00 00 00 00
scsi 2:0:0:0: [sdb] Assuming drive cache: write through
scsi 2:0:0:0: rejecting I/O to dead device
usb 5-8: new high speed USB device using ehci_hcd and address 4
usb 5-8: device descriptor read/64, error -110
usb 5-8: device descriptor read/64, error -110
usb 5-8: new high speed USB device using ehci_hcd and address 5
usb 5-8: device descriptor read/64, error -110
usb 5-8: device descriptor read/64, error -110
usb 5-8: new high speed USB device using ehci_hcd and address 6
usb 5-8: device not accepting address 6, error -110
usb 5-8: new high speed USB device using ehci_hcd and address 7
usb 5-8: device not accepting address 7, error -110

lsusb не показывает подключенное устройство, синий индикатор горит.

Если поднять pppd, то симптомы аналогичны, только секунд за 20 до отключение устройства синий индикатор гаснет, трафик не идёт.