How to put ads in header sidebar widget?

Site: https://rivalkingdomsfansite.com

I would like to move the google ads header which currently sits above the logo, next to it instead on the right into the large blank space. I assume that’s what the header sidebar widget is for but I’m not having any luck.

I tried using the text/html widget and placing it into the header sidebar and adding the google code, but it doesn’t display ads on all browsers so had to remove it. It seemed to work on mobile but not pc. What is the solution?

Thanks ??
Michael