Am I allowed to sit on my phone during my 30 minute lunch break?

My job legally requires 2 10 minute breaks and 1 30 minute lunch break, I only eat a granola bar so I just use my extra 25 minutes to chill on my phone.

But my manager walked into the break room and said “what are you doing?” And I said “I am on my lunch break” and they said “Well you are done eating so you should be going back to work!” so idk what to do,