Personally, I would start the design over with a theme that is specifically mobile responsive, but that is obviously not always the best option (or even an option at all in some cases). You might have luck with one of these plugins which claim to assist in making your website mobile responsive. You might look into using JetPack’s “Mobile Theme” option and see where that gets you.
Ref: https://www.searchenginejournal.com/5-fantastic-wordpress-plugins-make-site-mobile-responsive/111610/
Ref: https://jetpack.com/support/mobile-theme/
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