Can’t add additional slides

Hello,

I have tried many many times but an exisiting carousel slider won’t let me add additional slides.

There is no plus sign anywhere and the existing slides cannot be duplicated. There is no duplicate option from the 3-dot. And if I copy there is no way to paste. So how do I add a new slide?

I did a screen recording here: https://youtu.be/zwp9JyBpizI

The page I need help with: [log in to see the link]