This is much easier than it used to be.
firstly install the package usb-modeswitch then configure the settings for your ZTE modem in /etc/usb-modeswitch/19d2:2000:uPr=ZTE_CDMA and 19d2:2000:uPr=USB_ZTE
Just change all values for TargetProduct to TargetProduct= 0x0064 and comment out the TargetProductList.
In Network manager you’ll need to change "Edit Connections" and "Mobile Broadband" and tell the IPv4 section to be Automatic, instead of Automatic (addresses only)
That should be it – you’ll get some annoyance from the password box, I’m currently just entering anything in here – the letter ‘a’ seems to work sometimes, but it seems a bit random, maybe if you find the issue here you can let me know!