Header AD are not diplaying

Hello,
We encountered a problem with Header Ad. We need to display ads on desktop in one dimension (900x250px) and on mobile another (300x250px). We installed the whole Newspaper Theme template, after which we personalised it with Romania News template. After that we personalised the header with yet another template, called Header Template 10. The problem we came across is the visualisation of 900x250px header ad on desktop and ipad, and 300x250px ad on mobile devices. It is displayed on desktop and ipad, but disappears totally on mobile. We use Google AdSense for managing our ads. Can you please tell us how can we set the ad to make it display on mobile too?