Moin,

you could widerrufen the contract and send the iPhone back to O2 and oder anew one.

https://router-retoure.o2online.de/start/app/returnsorder

https://static2.o9.de/resource/blob/6952/ec5cb4a77e1a2abd0becb3187223399d/agb-widerrufsbelehrung-mobilfunk-privatkunden-download-data.pdf

Or you could ask for repaired.

https://www.o2online.de/service/repair/

Or if you have apple care, ask apple for support. If no apple care order apple care and ask Apple for support…

@Sheefoo70 

You're not the only one

https://discussions.apple.com/thread/255172109?sortBy=best 

Hello @Sheefoo70, 

welcome to our community :)

schluej has already given you some good tips. Have you already decided which way to go?

You can also find more information on repair services here: Dein O₂ Handy funktioniert nicht? Hier findest du Reparaturservices, Tipps, Anleitungen und Hilfe für dein defektes Smartphone | O₂ Community (o2online.de)

Best regards 
Cora