iPhone 5s keeps restarting randomly

I have an 32gb iPhone 5s which is only about 2 months old. It has suddenly started restarting randomly and did this about 4 times in the space of 30 minutes. Whenever I look up solutions, they tell me to update to ios 11.2 but my phone is already using 11.2.6 and can't be updated further.

Sometimes my phone turns off and won't turn back on for up to an hour. I know it's not a battery issue because it's been at 100% when it does this (and also it's a brand new phone so I don't see why that would be the issue). I've already tried to reset it and that's not working either. I know this isn't the right place to be posting this but I can't figure out where to put it if not here?

Are there any actual solutions to this problem or would my best bet be to take it to the apple store?