Mikrotik RB751G-2HnD with ZTE MF821D 3G/LTE USB Modem

I’ve a problem in configuring the ZTE MF821D 3G/LTE (4G) USB Modem (sold by the German carrier O2/Telefonica) with the RB751G-2HnD. Software version on the router is 5.20. The device is shown in /system resource usb print detail like this:

1 device="1:2" vendor="ZTE,Corporated" name="ZTE LTE Technologies MSM" vendor-id="0x19d2" device-id="0x0326" speed="480 Mbps" ports=0 usb-version="2.00"

But there are no ports shown in /port print.

The USB stick is already switched to modem mode. If I use the stick on Linux, I have to apply this commands:

modprobe option 
echo "19d2 0326" > /sys/bus/usb-serial/drivers/option1/new_id

Afterwards I can use the /dev/ttyUSB2 to issue commands to the device:

ati
Manufacturer: ZTE Corporation                                                  
Model: MF821D
Revision: M9200B-SCAQDBZD-3.3.330115T  1  [May 06 2012 03:00:00]
IMEI: xx1311010xxxxxx                                                         
+GCAP: +CGSM,+DS,+ES

Can anybody help me out? Thanks.