Thank you so much for using the support forum for the queries.
While visiting your website we got the same thing that you stated above. However, we cannot know what you actually did on the backend. Hence, For making the home page, you need to set a static front page. And you said that it is already set.
As we see that,
http://preci-dental.be/en/welcome/ page is selected as a home page. Check once by changing the page on static front page. Please check once and let us know.
To hide the menu, you can disable the front page content from the customizer setting. Please check all the available customizer options inside Appearance > Customize.