What do i say when i want to say i'm back online after being offline during a long period of time? Can i use 戻りました and/or 帰って来ました?
Yes, both are fine. Both convey your feeling that whatever online forum you are coming back to is your "home."
Both expressions also convey a certain amount of respect, and as a result they sound a bit formal. It's a kind of expressions that you can use to seniors, teachers, colleagues, etc. If you are aiming for more informal expression to friends, I suggest 戻ってきた(よ) or 帰ってきた.