Successfully enabled #CellBroadcast in #Mobian forky/sid on my #oneplus6t. Thanks @agx for your amazing work!
Turns out that the stable upstream ModemManager release doesn’t support it on the OP6T yet, so I had to pull both `modemmanager` and `libmm-glib0` packages from `experimental`, which upgraded ModemManager to the development release.

Added `/etc/apt/sources.list.d/debian-experimental.sources`:

Types: deb deb-src
URIs: http://deb.debian.org/debian/
Suites: experimental
Components: main non-free-firmware
Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg

And `/etc/apt/preferences.d/90modemmanager`:

Package: modemmanager
Pin: release a=experimental
Pin-Priority: 800

Package: libmm-glib0
Pin: release a=experimental
Pin-Priority: 800

Index of /debian

And to be safe, `/etc/apt/preferences.d/99experimental`:

Package: *
Pin: release a=experimental
Pin-Priority: 1