Rotating my iPhone 📲 8 Plus

How can I rotate my iPhone 8Plus to be able to read horizontally as well as vertically?

I'm almost certain I was able to do this when I got this iPhone, but at some point it stopped. I thought I could correct it by going to Settings, but I couldn't find anything that would help. This is my 3rd iPhone and both of them would rotate all the time.

Can you help me.