Mobile data is now implemented in the close-to-mainline kernel port! SMS also seems to work, but this may depend on the provider since there is no VoLTE support yet. Only the first SIM slot works, but this will be fixed later.
For those who are interested, here are the changes I had to make to oFono: Comparing sailfishos:master...affenull2345:unisoc · sailfishos/ofono · GitHub
To avoid having to keep old images on my server, there will only be a single link to the latest image from now on: https://storage.abscue.de/private/zImage/Sailfish_OS-s19mps-latest.zip
The installation process was simplified so that the image can now be downloaded and installed directly on the Jolla C2. The kernel no longer needs to be built separately. After downloading the image, just run the following commands on the phone (make sure an SD card is inserted and there is no important data on it):
unzip SailfishOS-5.0.0.62-s19mps-0.0.1.202503152044.zip
cd SailfishOS-5.0.0.62-s19mps-0.0.1.202503152044/
./install-on-device.sh
See the localbuild repository linked in the previous post for more information about booting back to the original Sailfish OS.