I hope someone can help me with this issue. Thank you so much!
The store link that you have provided to us is password protected. So, kindly share the password so that I can take a look and can help you.
To make the sticky header you will need to add the custom JS and CSS to your store theme files. I can assist you in doing so if you can share the access with me.
You can DM me the credentials so that will take a further look and will work on it.