Xperia 1 II stuck in flight mode. Disconnect battery helps momentarily

When trying to make a call, a friends Xperia 1 II displays "turn off flight mode", even though flight mode is not turned on.

Toggling flight mode, re-boots, Safe mode, changing SIM cards, reset network settings, toggling wifi calling, does nothing.

What does work though, is disconnecting and reconnecting the battery. But only after the first start up after the disconnection. As soon as I reboot the phone a second time, the problem comes back...

A diagnostic test reveals UICC failure.

Google says UICC has something to do with the SIM. So my thinking is that some component on a part of the main board that handles network stuff got (water) damaged through a broken screen. And during the startup process it gets flagged as flight mode. Or something like that...

But that doesn't make sense, since everything works fine after the first startup after a disconnection. And that makes me think it can be fixed.

Any takers?

It's stock firmware. So I don't really think that a reinstall would solve anything. But I might be wrong?

Also. I can't get the unit into recovery mode, to clean the system cache. Don't know if that would make a difference though?