"You must bring a standalone camera if you want to take pictures..."
Original sentence: "You must bring a standalone camera if you want to take pictures. You cannot bring your mobile phone inside."
Proposed translation: "Du/dere må bringe et stående kamera om du vil ta bilder. Du/dere kan ikke bringe mobiltelefonen din innsid."
If I want to combine both sentences (...take pictures because you cannot bring...): "Du/dere må bringe et stående kamera om du vil ta bilder fordi du/dere kan ikke bringe mobiltelefonen din innsid."
I was wondering if there is a better way or if this is fine.
Inside would often be translated as innendørs (indoors) if you talk about a museum, concert hall or similar and other than that I think I would phrase your first suggested sentence something like this: "Du kan medbringe et kamera om du vil ta bilder innendørs, så lenge det ikke er en mobiltelefon". But in Norwegian it would be rather common to say that something is forbudt, so another alternative would be to write: "Du kan medbringe et kamera om du vil ta bilder inne, men det er forbudt å bruke mobilkamera/mobiltelefonens kamera."
Edit: With this translation you would tell people they cannot use the camera of the cell phone, rather than it being against the rules to bring the cell phone inside at all, so a slight difference from the original sentence you quoted.