living in Bern, working in germany freelance

HI ,

my situation is a bit special, I'm a professional opera singer living in Bern. Brexit has made things very complicated !

Some German opera houses are making it difficult to work in their theatres , saying I need a visa or a work permit.

I left the UK 27 years ago and have lived in Switzerland for 20 years already, I have a C permit.

is it true that I can work in Germany , as long as it's not more than 90 days in a 180 day period?

please help me, no-one seems to know the new rules, and all the theatres react differently.

best

Rob