Computer and tablet addiction?

My daughter is 12 and spends practically all day on her tablet or laptop,  I have tried turning the internet off for some of the day but it causes so many arguments,  she's moody when not playing her favourite online games, she shouts at me and her brother.  She literally sits on her bed all day o lying coming out for her dinner.   Does anyone have any advice, or know where I can go for help please?  I cant stand watching her wasting her life away, doing nothing but playing online games. Thankyou