How do I use my old iphone as a mp3 player?

I have used my iPhone5 for years as an mp3 player. When I powered it up, I would not login to iTunes because the music is stored in the phone. Recently the phone started and went into activating the cell service. I cannot bypass that and return to using the stored music. How do I bypass cell service activation? If there is no hope of using it, maybe I should stop paying to store my data.