I finally suceeded in the downgrade of my Xperia 10 from Android 10 to Android 9. In the End of the tutorial https://jolla.zendesk.com/hc/en-us/articles/115004283713 you shall check whether everything is working. My Sim Card can be found. I can unlock the phone with the pin number and they show my sim card with the correct provider. But it shows a wrong phone number and neither phone calls nor mobil data are working. I tried another Android 9 Version on Emma but it did not work either. Has anybody an idea what i can try next? 
What does this mean? Where?
Phones don’t actually know their on phone number… These that claim to use a special phonebook entry on the SIM-card.
Okay so maybe this is not part of the problem but the real problem is that I have “no service”, so I can do no phone calls and have no mobile data.
Have you tried to put other SIMs in the device?