Despite I tested a lot of LTE-Modems for ModemManager and NetworkManager. I never knew what did happen behind it.
#ipv6
here Aleksander Morgado explains it
https://modemmanager.org/docs/modemmanager/ip-connectivity-setup-in-lte-modems/
IP connectivity setup in LTE modems
An outline of the different IP settings a ModemManager bearer object may expose, and the expected actions the host would require to perform to set up the IP addressing in the modem.modemmanager.org