There exist topic for this device (in Russian).
If your device have wifi you can switch off wifi with command:
echo "at+wifi=0" > /dev/ttyUSB1
Then try /dev/ttyUSB0 in PPP mode again.
hej "Zap", I don't think my problem applies to your case.
my mode is :6000 which seems to requires the option serial driver.
I'm assuming your case requires the rndis kmod installed.
it should load automatically upon plugin. if your are certain you rndis is the correct driver just follow AndrewZ steps above, replacing /sys/bus/usb/drivers/qmi_wwan/new_id with the rndis path.
Good luck (i'm obviously not the expert here)