Make widgets appear only at defined screensize?

Would use CSS Media Queries for this, see e.g. here:
https://cssmediaqueries.com/what-are-css-media-queries.html

Your responsive Oxygen theme already uses such media queries to relocate certain elements for different screen sizes, see Media Queries (mobile browsing) in your file oxygen/style.css.

With that theme you would change the main html/css code at several places or you could (as a quick workaround which only changes style.css) add a second text-widget with the same contact-info (which on normal screens gets hidden via css display: none;) to #sidebar-secondary and add some media query css which then hides the text-widget in #sidebar-primary on smaller screens and shows the hidden contact in seconday instead.

A plugin like Widget Logic would be the wrong approach for this case.

Hello, and thank you for your answer.

I see your point, but if I create a second text-widget to, lets say sidebar-primary, how do I hide only that, and not the entire sidebar-primary widget? If I add display:none to #sidebar-primary, obviously all text-widgets within that widget, will be hidden.

Can a hide a specific text-widget by adding maybe the widget-title or something, to #sidebar-primary {dislplay:none} in the css?

Again, thank you very much for helping out, I appreciate it.
Kristian

Don’t hide the whole sidebar, only the text-widget. It has an own id which you can see e.g. in the html source.

This would hide your “REEL” Widget:

#sidebar-secondary #text-4 {
    display: none;
}

Aah, I see, they’ve got individual div-IDs

That’s the information I needed, thank you very much.

Cheers.