Sticky Header for All Site

Hello,

How can i made my header slide down (sticky) in all pages ?

I check documentation: https://blockpresswp.com/how-to-make-sticky-and-transparent-header/ it works in main page. But for other pages and posts not work correctly.
For this post, it works correctly: https://blockpresswp.com/how-to-make-sticky-and-transparent-header/

My main page: https://www.porsukmedya.com/ scrolling header is working good.
Any other page or post: https://www.porsukmedya.com/hizmetlerimiz/
Image examples in here: https://imgur.com/a/gFv4KBj

Thanks for your help.

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