Strange: today, WWAN is back. No changes to the system. Just reboots. But I DID reboot before to recover it. Didn't work.
What I realized: The modem was auto-enumbered as "1" (when it did not work), now it's back to "0" (and works), like in
mbimcli --device=/dev/cdc-wdm0 --device-open-proxy --query-device-caps