@zimma71 hello,maybe,if you haven't tried Reset network settings,you could try it.Go to Settings/General management,Reset-Reset network settings.You don't lose personal data,but you have to set up wireless connections again.
Check if you have established applications only with mobile data! This setting is found in Settings / Connections / Data usage. If you have established any app that uses only mobile data - delete it and check again. Greetings.