With recent extensions and the upgrade to Linux 5.10 it is a good idea to switch from the plain old AT commands for configuring a wirelessmodem session to the QMI/MBIM interface which also supports "raw ip".
"raw ip" means that any addressinf will be done directly on the operating system managed wwan0 interface. This interface can be treated like an eth or wlan interface, thus we can also completely reuse the vyos.ifconfig class.