Problem with a calculator im building

Hi! Im making a iphone imitation calculator, I used a freecodecamp tutorial to help myself out and Its going pretty well! But there is a problem where after the first time you load the page and put in numbers, you can only put in one number, they dont add on to each other making a sequence of numbers. Im not sure why this is happening because the first entry of numbers after loading it works fine, but after that the display just replaces with whichever number is pressed. I would love some help! Thank you:)

here is a link to the codepen of my project: