Sorry for long silence, but we were trying to recreate the issue, with different devices, and on couple ones there was problem with native network manager.
what you can try is:
First of all you will have to disable raspbian default network manager with command:
- systemctl stop networking.service
- systemctl disable networking.service - this will prevent default network manager from starting again automatically
And for some reasons default raspbian dhcp service do not work properly with modems, so you will have to use udhcpc.
- install udhcpc sudo apt install udhcpc
- request ip address with udhcpc -i wwan0
With these two changes you will have to get internet connection via mobile interface.
Hope this helps.